Rename all functions to use "_" instead of "."

[GegznaV]

Rename all the functions like read.txt.Witec() into read_txt_Witec(). In particular, function names should be the same as filenames that contain those functions, e.g., in https://github.com/r-hyperspec/hySpc.read.Witec/blob/develop/R/read_txt_Witec.R
https://github.com/r-hyperspec/hySpc.read.Witec/blob/16a57f09b1c149e43d3f31d9284f740bc808e825/R/read_txt_Witec.R#L24

There are several not renamed functions including read.mat.Witec(), read.dat.Witec(), and read.txt.Witec().

[eoduniyi]

Created the branch feature-12-rename.2_ to take care of this issue

[bryanhanson]

@cbeleites @eoduniyi has completed this issue but I was wondering if this change requires some kind of deprecation. If no, please close this issue.

[GegznaV]

For exported functions only, these changes should be mentioned in NEWS.md in hyperSpec repo.

[GegznaV]

And when issue cbeleites/hyperSpec#263 is solved, I'll add proper deprecation messages to relevant functions in hyperSpec (as I did with hySpc.ggplot2).

[eoduniyi]

PR #26 closes this issue