Several small improvements

[benkrikler]

Added

• New postprocessing stage to filter columns
• New option to AssignCols stage to make assignment the index

Fixed

• ToDatacardInputs had been broken in a previous update

[codecov[bot]]

Codecov Report

Merging #34 into master will not change coverage. The diff coverage is 0.00%.

@@          Coverage Diff           @@
##           master     #34   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files           8       8           
  Lines        1120    1139   +19     
======================================
- Misses       1120    1139   +19     

Impacted Files	Coverage Δ
fast_plotter/postproc/functions.py	0.00% <0.00%> (ø)
fast_plotter/postproc/stages.py	0.00% <0.00%> (ø)
fast_plotter/version.py	0.00% <0.00%> (ø)