Make GRIB2 packing easier

[EricEngle-NOAA]

Currently, users must define the GRIB2 packing method when creating a Grib2Message object either by passing the entire section5 array or with drtn keyword argument.

Instead, lets attempt the following:

• packing method strings that map to a DRTN (i.e. simple = DRTN 0)
• default packing scheme (likely complex packing w/ 2nd order spatial differencing)
• update Grib2Message.pack() method to support this. Currently, it just triggers the packing calls and expects the section5 information to be present.

[EricEngle-NOAA]

337d339 provides the bulk of the changes for this issue.

The Grib2Message object now defines an attribute packing which defaults to None, but when packing information has been provided, it is an instance of Grib2Packing class. This dataclass holds section5 and attributes.

New method, set_packing() performs the create of the dynamically inherited Grib2Packing. The default packing will be complex packing with spatial differencing (DRTN 3). User can provide the packing with the template number or a string representing the template number.

Packing Scheme	Integer Value
'simple'	0
'complex'	2
'complex-sd'	3
'ieee-float'	4
'jpeg'	40
'png'	41
'aec'	42
'spectral-simple'	50

User can also pass keyword arguments of mutable packing options. Each data representation template class now contains _mutable tuple of attribute names that are configurable.

When reading GRIB2 messages from file, section5 array can still be passed to Grib2Message and the section5 attribute is now a propery that returns Grib2Message.packing.section5