Included si5351 routines

• Included Jerry Gaffke's, KE7ER, "minimalist standalone si5351bx routines".
• This not only makes the sketch independant from an external SI5351 library, but it greatly reduces the memory usage as well. The program space is needed for future development of additional features that would otherwise not fit in a Nano. Thanks Jerry!

Improved tuning pot responsiveness

• Improved responsiveness of the tuning pot for smoother tuning (less need to fiddle up and down to set the correct frequency)
• Improved "Fast Tune" (at either ends of the tuning pot). The step size is now variable: the closer to the pot limit, the larger the step size.
• In CW SPOT tuning mode, short side tone pulses will be generated instead of a continuous tone. This makes SPOT tuning easier when tuning to weak CW signals.
• Calibration can now done at 1 Hz precision

Improved CW performance

• improved CW performance at higher CW speeds:
  reduced the delay at the start of CW transmissions (first dit is no longer lost at high speed CW)
  optimized code so that 1:3 CW-ratio is kept even at high CW speed
• improved FINE TUNE mode so that exact frequency (at 1 Hz precision) is displayed while the SPOT button is held pressed
• added an extra option in the SETTINGS menu for setting semiQSK ON or OFF.
  This may be useful for CW operators who want to manually activate the PTT (e.g. using a foot switch).
  if semiQSK is ON:
  radio will automatically switch to CWL (or CWU), and go into TX mode when the morse key goes down
  go back to RX automatically when the QSKdelay time is exceeded
  radio will switch back to LSB (or USB) when the operator presses the PTT switch
  if semiQSK is OFF:
  operator must activate PTT manually to move the radio in TX
  pressing the PTT does not affect the mode. Use the Function Button to select the desired mode (LSB-USB-CWL-CWU)
• corrected a bug that FINE TUNE was not properly applied in USB mode

corrected bug in shiftBase() routine

corrected bug in shiftBase() routine that the radio didn't return to the correct frequency after switching VFO's, SPLIT, RIT, FINE TUNE, etc.

Added FINE TUNE capability to the SPOT button

• Added "Fine Tune" capability to SPOT button
  While the SPOT button is held pressed, the radio will temporarily go into "FINE TUNE" mode, allowing the VFO to be set at 1Hz precision. This feature works also in SSB mode (except that no sidetone will be generated then).

Added CW SPOT tone functionality

Connect a pushbutton to Arduino pin D4. A SPOT tone will be heard when D4 is connected to ground.
By aligning the CW Spot tone to match the pitch of an incoming station's signal, you will cause your signal and the other station's signal to be exactly on the same frequency.

small bug correction (RIT control in CW mode)

RIT offset should only be applied during RX. This is indeed the case in SSB mode. But in CW mode, RIT offset was not turned off during TX. This has been corrected - RIT works correctly now in all modes.

Added true RIT functionality

Added true RIT functionality (adjustable RX offset) (2 Function Button presses)
The existing 'RIT' function, based on switching VFOs, is now called "SPLIT" (3 presses)
Mode selection (4 presses) can now also be used to select CWL and CWU
Major code cleanup to reduce memory usage
Inserted some delay in various routines to prevent buzzing sound in SETTINGS menu

small bug correction

Corrrected a bug that the display was constantly refreshed when the radio was idle. This caused constant ticking sound.

implemented some CW improvements, added A/B monitoring mode

• added VFO A/B monitoring mode (press Function Button 5 times)
• use RX-offset instead of TX-offset in CW mode - the display now shows the correct TX frequency in CW
• changed the way to switch from CW to SSB mode: press PTT to return to SSB mode (tks Willy W1LY)
• restored the functionality for old way calibration method
• simplified the method for sidetone setting: hold key down to hear sidetone
• improved the display during "fast scan" at tuning pot limits (tks Paul KC8WBK)