Rename plot flags in plot2D and add deprecation warning

[oskooi]

Renames the keyword parameters plot_eps_flag, plot_sources_flag, plot_monitors_flag, and plot_boundaries_flag of plot2D to show_epsilon, show_sources, show_monitors, and show_boundary_layers, respectively. Also adds a deprecation warning. This renaming is intended to improve readability.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 60.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 73.71%. Comparing base (f29a8c7) to head (373bd41).
Report is 30 commits behind head on master.

Files	Patch %	Lines
python/visualization.py	60.00%	8 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2885      +/-   ##
==========================================
- Coverage   73.81%   73.71%   -0.11%     
==========================================
  Files          18       18              
  Lines        5423     5444      +21     
==========================================
+ Hits         4003     4013      +10     
- Misses       1420     1431      +11     

Files	Coverage Δ
python/simulation.py	77.41% <ø> (ø)
python/visualization.py	37.31% <60.00%> (+0.28%)	⬆️

[smartalecH]

LGTM!