Adding tests for the restart mechanics

[llaniewski]

This adds tests for the restarting mechanics for d2q9. Tests both vtk files as well as csv.

Also adds a simple tool for merging csv files from restart runs.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f7a0f5f) 43.23% compared to head (6da3bc1) 35.14%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #483      +/-   ##
==========================================
- Coverage   43.23%   35.14%   -8.09%     
==========================================
  Files         166      162       -4     
  Lines        7536     6621     -915     
==========================================
- Hits         3258     2327     -931     
- Misses       4278     4294      +16     

Flag	Coverage Δ
d2q9	31.92% <ø> (+1.46%)	⬆️
d3q27_PSM_NEBB	?
d3q27_pf_velocity	?

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