A few fixes related to usage as a library

[ml-evs]

This PR mostly contains fixes that allow this package to be used as a library (i.e. non-editable install).

• Change optimade.config to look in the cwd instead of optimade install directory for server.cfg file and moved the server_template.cfg into the server directory so it can be included as package data.
• Removed package_data from setup.py in lieu of explicitly specifying all test data and associated config files in MANIFEST.in.

Aside from that, it fixes some small issues:

• stop the single structure endpoint returning data_returned: 1 when the structure is not found.
• check all test paths when mocking
• do an editable install when running pre-commit for CI: some strange behavior here changes the openapi spec depending on the install method.

[codecov]

Codecov Report

Merging #119 into master will decrease coverage by 0.08%.
The diff coverage is 64.28%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage   82.81%   82.73%   -0.09%     
==========================================
  Files          39       39              
  Lines        1810     1813       +3     
==========================================
+ Hits         1499     1500       +1     
- Misses        311      313       +2

Flag	Coverage Δ
#unittests	82.73% <64.28%> (-0.09%)	⬇️

Impacted Files	Coverage Δ
optimade/server/entry_collections.py	83.96% <100%> (ø)	⬆️
optimade/server/main.py	91.83% <33.33%> (-1.92%)	⬇️
optimade/server/main_index.py	89.47% <33.33%> (-2.2%)	⬇️
optimade/server/config.py	76.54% <80%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8510ff5...70074eb. Read the comment docs.

[CasperWA]

* Change `optimade.config` to look in the cwd instead of optimade install directory for `server.cfg` file and moved the `server_template.cfg` into the server directory so it can be included as package data.

This should be possible also with the MANIFEST.in file. But for the template file, it is of no importance where it is, as long as the server.cfg is created somewhere easily accessible.

Aside from that, it fixes two small issues:

* stop the single structure endpoint returning `data_returned: 1` when the structure is not found.

Huh, thought I fixed that earlier - probably missed it. Thanks! 👍

[CasperWA]

Great work @ml-evs!

Have you tried to test if the package data is included with the new setup?

[CasperWA]

I have updated the README with information on where to put server.cfg and have made sure it also works (at least for starting up the servers - I trust GH Actions to check the inner workings of the servers.

[CasperWA]

This is firing on all cylinders now, thanks @ml-evs!