TODOS for read_txt_Witec_ASCII

[cbeleites]

• [x] find out better name - waiting for Ute's reply
• [x] provide axis labels
  • [x] x axis units are in $SpectrumHeader$XDataKind
• [x] allow users to specify extra data to keep
• [x] unit testing
• [x] documentation

[eoduniyi]

I think Claudia and I cleared everything off of this list. Closing this now.