Set up using custom JSON decoders/encoders

[CasperWA]

Closes #79

This will retrieve JSON encoders from POSTed data within the configurations, and JSON decoders from URL query parameters.

[CasperWA]

@jesper-friis I have implemented the way I think custom JSON encoders and decoders should be retrieved for the /filter router. I hope we can use this as a grounds of discussion concerning whether this is the way to do things or not - after we agree it can be reproduced in the rest of the code base.

[codecov-commenter]

Codecov Report

Merging #96 (ba314f0) into master (1ae3b76) will decrease coverage by 1.17%.
The diff coverage is 55.17%.

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
- Coverage   69.87%   68.69%   -1.18%     
==========================================
  Files          17       17              
  Lines         468      492      +24     
==========================================
+ Hits          327      338      +11     
- Misses        141      154      +13     

Flag	Coverage Δ
pytest	68.69% <55.17%> (-1.18%)	⬇️

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

Impacted Files	Coverage Δ
app/routers/datafilter.py	68.57% <55.17%> (-11.87%)	⬇️

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[CasperWA]

This, I think, can now be closed. The idea of custom JSON decoders/encoders is old and should be reconsidered. The issue of whether it should be included in the first place should first be discussed before implementation suggestions pop up.