Add sortable=True to all properties

[CasperWA]

Fixes #273

Since for MongoDB all properties can be sorted, it is done like this.
If one uses a different backend, one should overwrite the utility
function and use a different method to determine the sortability of
properties.

Slightly updated the wording of the sortable model attribute to be
more accurate: It is only a MUST if sort is supported AND the given
property can be used for sorting.

[codecov]

Codecov Report

Merging #274 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #274   +/-   ##
=======================================
  Coverage   89.92%   89.93%           
=======================================
  Files          54       54           
  Lines        2244     2245    +1     
=======================================
+ Hits         2018     2019    +1     
  Misses        226      226           

Flag	Coverage Δ
#unittests	89.93% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
optimade/models/entries.py	100.00% <ø> (ø)
optimade/server/routers/utils.py	97.72% <100.00%> (+0.01%)	⬆️

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[ml-evs]

I responded over on the issue, but maybe its more useful to comment on this here.

I'd prefer a conservative approach which allows database providers to specify sortable fields as configuration options (i.e. sortable=False by default). I'm not sure the effort of supporting multiple sort fields is worth it, at least not with the current OPTIMADE properties (in which case we need to throw an error)?

EDIT: I actually read your changes... this looks fine to me in principle, but think it should be configurable, as stated above.

[ml-evs]

Thanks @CasperWA, this is a useful change to improve our compliance with the spec!