Check if functions return correct `spc` column names

[GegznaV]

Add unit tests for spc_*() and related functions that modify $spc to test if they return correct column names of $spc. Where needed, the output should be fixed to return correct column names in the $spc matrix. For details, see:

• cbeleites/hyperSpec#237
• cbeleites/hyperSpec#245

Tasks:

• [ ] Create a list of functions that modify $spc
• [ ] Create unit tests for each function

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(Incomplete) List of functions:

• [ ] - 1. .spc_fix_colnames
• [ ] - 2. .spc_io_postprocess_optional
• [ ] - 3. spc_bin
• [ ] - 4. spc_fit_poly
• [ ] - 5. spc_fit_poly_below
• [ ] - 6. spc_loess
• [ ] - 7. spc_na_approx
• [ ] - 8. spc_rubberband
• [ ] - 9. spc_smooth_spline
• [ ] - 10. vanderMonde
• [ ] - 11. ...

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This originally was issue

• https://github.com/cbeleites/hyperSpec/issues/248