Updates to plotting functions

[jdebacker]

This PR makes the following updates:

• Use 300 dpi when saving plots to disk
• Better labels of the plot_industry_aggregates plots

[codecov-commenter]

Codecov Report

Merging #857 (be52d62) into master (4fb68cb) will decrease coverage by 0.02%.
The diff coverage is 80.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #857      +/-   ##
==========================================
- Coverage   82.18%   82.17%   -0.02%     
==========================================
  Files          18       18              
  Lines        3992     3999       +7     
==========================================
+ Hits         3281     3286       +5     
- Misses        711      713       +2     

Flag	Coverage Δ
unittests	82.17% <80.55%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
ogcore/constants.py	100.00% <ø> (ø)
ogcore/output_plots.py	96.49% <76.47%> (-0.45%)	⬇️
ogcore/parameter_plots.py	80.99% <84.21%> (ø)

[rickecon]

@jdebacker. Looks great. Merging.