Use `file` for file name input

[GegznaV]

In read_*() functions, we currently use file, files, filename or con to indicate the path to file or connection. We should use file consistently. Update argument name in these functions:

• read_asc_Andor
• read_asc_PerkinElmer

• read_ini

• read_txt_Horiba
• read_txt_Horiba_t
• read_txt_Horiba_xy

• read_zip_Renishaw
• read_txt_Renishaw

• read_txt_Shimadzu

• read_dat_Witec
• read_txt_Witec
• read_txt_Witec_Graph
• read_txt_Witec_TrueMatch

NOTE: some functions use specific argument names like filex. Please, mention functions like this by commenting on this issue. We should decide on the names of these arguments separately.

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• ⛔ Issue Review, add and improve unit tests #42 should be solved first and unit tests must work properly.

[GegznaV]

Might be helpful:

• Code from fileio.Rmd vignette for unit tests cbeleites/hyperSpec#174

[GegznaV]

This issue is ready to be solved.