Pydantic response models

[CasperWA]

Closes #9
Closes #51
Closes #47

The latest commits expect the new SessionUpdate object is returned from strategy methods.

Note: This is a PR opened by me from the work done by @pscff to implement pydantic data models for the API responses and more. I have done a full overhaul in the latter commits to fully finish and implement the ideas started by @pscff.

[codecov-commenter]

Codecov Report

Merging #44 (5e678ed) into master (3e57cd0) will decrease coverage by 0.92%.
The diff coverage is 62.89%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
- Coverage   69.49%   68.57%   -0.93%     
==========================================
  Files           8       15       +7     
  Lines         295      420     +125     
==========================================
+ Hits          205      288      +83     
- Misses         90      132      +42     

Flag	Coverage Δ
pytest	68.57% <62.89%> (-0.93%)	⬇️

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

Impacted Files	Coverage Δ
app/routers/dataresource.py	29.41% <19.29%> (-3.38%)	⬇️
app/routers/session.py	50.74% <43.18%> (-6.67%)	⬇️
app/main.py	78.37% <57.14%> (+2.70%)	⬆️
app/routers/transformation.py	68.65% <63.63%> (-13.17%)	⬇️
app/routers/datafilter.py	80.85% <73.52%> (-11.26%)	⬇️
app/routers/mapping.py	80.85% <75.67%> (-11.26%)	⬇️
app/models/error.py	85.71% <85.71%> (ø)
app/models/response.py	92.85% <92.85%> (ø)
app/models/datafilter.py	100.00% <100.00%> (ø)
app/models/dataresource.py	100.00% <100.00%> (ø)
... and 5 more

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[CasperWA]

@pscff I'll stop messing around here now - feel free to comment on whether you want to implement further changes (or simply do) in the scope of this PR :)

[CasperWA]

@quaat and @jesper-friis There is some confusion on my side as to the dataresource endpoint, when handling a download and parse strategy. For the get() method it seems straight forward, but at the moment parse strategies' initialize() method was never called. I've implemented this here now, but I'm unsure about the exact nature of this - please review.
In my opinion it would be cleaner if only a parse strategy was invoked, but that would implicitly mean all parse strategies should make sure to invoke a download strategy internally; this doesn't seem reasonable. But then should the parse strategy be initialized first or the download strategy? Hmm, bah.