Some plotting fixes to science plotting

[hayesla]

This PR fixes:

• returns im rather than axes for plot_spectrogram, which allows user to then do plt.colorbar of fig.add_colorbar(im)
• fixes #129
• fixes #130

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.

Project coverage is 73.50%. Comparing base (1b41df0) to head (1484e46).

Files	Patch %	Lines
stixpy/product/sources/science.py	0.00%	15 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #134 +/- ## ========================================== - Coverage 73.98% 73.50% -0.48% ========================================== Files 32 32 Lines 1999 2012 +13 ========================================== Hits 1479 1479 - Misses 520 533 +13 ```

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