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 Compressed pixel data plotted in units of cts/cs/keV with plot_timeseries() #129
• fixes Give units to energy bins in legend for cpd.plot_timeseries()  #130

[codecov]

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 ⚠️

Additional details and impacted files

@@            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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