obs_sequence object aware whether it has assimilation results or not

[hkershaw-brown]

• Added two bits of information to the obs_sequence class:
  self.has_assimilation_info = False
  self.has_posterior = False

• An obs_sequence object is then aware of whether it has assimilation info (prior only or prior and posterior)

• Added decorator for class methods that require assimilation to check info is present.
  Select by qc and possible vs used functions now class methods and use the decorator (because they need assimilation info)

• Now have a posterior bias and sq error column (note I've not renamed bias & sq error to prior_bias, prior sq error)

• Added tests for requires assimilation_info, init method changed so you don’t need a file for testing the data frame routines.

• Added an early exit if no file is given to obs_sequence::init. This is so you can test the data frame methods without worrying about having a file to give data.

fixes #25
fixes #27
fixes #19

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.09091% with 7 lines in your changes missing coverage. Please review.

Project coverage is 74.38%. Comparing base (ea68901) to head (f8542cc).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pydartdiags/obs_sequence/obs_sequence.py	84.09%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
+ Coverage   72.12%   74.38%   +2.26%     
==========================================
  Files           2        2              
  Lines         495      527      +32     
==========================================
+ Hits          357      392      +35     
+ Misses        138      135       -3     

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