Create function `wl_create_label_from_units()`

[GegznaV]

Basing on the lines from R/wl_convert_units.R (see this comment ):

  x@label$.wavelength <-
    switch(.wl_fix_unit_name(to),
       nm    = expression("Wavelength, nm"),
      invcm = expression(tilde(nu) / cm^-1),
      ev    = expression("Energy / eV"),
      freq  = expression(nu / THz),
      raman = expression(Raman ~ shift / cm^-1),
      to
    )

A separate function should be created. The function's signature could be wl_create_label_by_units(wl_units) wl_create_label_from_units(wl_units) and it should return an expression with label for @label$.wavelength.

If this is implemented, labels in

• https://github.com/r-hyperspec/hyperSpec/issues/52 are not necessary and the related .R file can be deleted.

---

Tasks:

• Add unit tests for wl_convert_units.hyperSpec() to test if labels are correct in all cases of to values. Moved to #69
• Create a new function #61
  • [x] Create a new function
  • [x] Create unit tests for the new function
  • [x] Update NEWS.md
• [x] Delete constants-units.R and close #52

[cbeleites]

is by the correct preposition? @bryanhanson ?

[bryanhanson]

"by" is OK, but "from" is better.