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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• [x] ⛔ Issue #42 should be solved first and unit tests must work properly.

[GegznaV]

Might be helpful:

• https://github.com/cbeleites/hyperSpec/issues/174

[GegznaV]

This issue is ready to be solved.