deprecate/remove output.path argument from fit_CWCurve()

[mcol]

There's this TODO note: https://github.com/R-Lum/Luminescence/blob/759a1142651cc557f9fd06c6b061d1aff244cdf9/R/fit_CWCurve.R#L224-L225

As now with more coverage we hit the lines where output.path is used and an output CSV file is generated, we get a note when running R CMD check:

 checking for detritus in the temp directory ... NOTE
  Found the following files/directories:
    ‘RtmpZkat2Ifit_CWCurve_Output_Default.csv’

So perhaps it's a good reason to actually remove that argument?

[RLumSK]

@mcol: Yes, I guess this is an ugly left-over before RLum-class object time. I suggest setting it to defunct directly and then we remove it in a version 1.x.x.

[mcol]

I take that you mean to remove the code altogether and for the time being throw a warning if somebody were to set the argument, but just say if you meant something else.