Fix showing and saving plot in check_swatinit

[larsevj]

Resolves #795

From https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.show.html:

Notes

Saving figures to file and showing a window at the same time

If you want an image file as well as a user interface window, use [pyplot.savefig](https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.savefig.html#matplotlib.pyplot.savefig) 
before [pyplot.show](https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.show.html#matplotlib.pyplot.show). 
At the end of (a blocking) show() the figure is closed and thus unregistered from pyplot. 
Calling [pyplot.savefig](https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.savefig.html#matplotlib.pyplot.savefig) afterwards would save a new and thus empty figure. 
This limitation of command order does not apply if the show is non-blocking or if you keep a reference to the figure and use [Figure.savefig](https://matplotlib.org/stable/api/_as_gen/matplotlib.figure.Figure.savefig.html#matplotlib.figure.Figure.savefig).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.16%. Comparing base (c174caf) to head (6956132).

Files with missing lines	Patch %	Lines
src/subscript/check_swatinit/check_swatinit.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #807   +/-   ##
=======================================
  Coverage   85.16%   85.16%           
=======================================
  Files          49       49           
  Lines        7544     7544           
=======================================
  Hits         6425     6425           
  Misses       1119     1119           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.