Updates to plotting functions

[jdebacker]

This PR updates some plotting functions and tests of them to avoid deprecation warnings. Partly addresses Issue #951.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.01%. Comparing base (92f7594) to head (8966208).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
ogcore/parameter_plots.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #978      +/-   ##
==========================================
+ Coverage   69.98%   70.01%   +0.02%     
==========================================
  Files          20       20              
  Lines        4977     4985       +8     
==========================================
+ Hits         3483     3490       +7     
- Misses       1494     1495       +1     

Flag	Coverage Δ
unittests	70.01% <95.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
ogcore/output_plots.py	88.99% <100.00%> (+0.07%)	⬆️
ogcore/parameter_plots.py	78.30% <92.30%> (+0.01%)	⬆️

[jdebacker]

@rickecon This PR is ready for your review. No version bump here -- just updating to avoid warnings.

[rickecon]

@jdebacker. LGTM. Merging.