Rework models code to use the backend, and other improvements to it

[mhauru]

This removes all the bits from both the Arima and HODMD model codes that directly read from/wrote to the database, and replaces them with calls to the backend API. While I was at it and learning my way around the code I also made other small changes.

The diff can be made nicer once #175 has been merged.

Closes #82

[codecov-commenter]

Codecov Report

Attention: 31 lines in your changes are missing coverage. Please review.

Comparison is base (6cff24b) 88.40% compared to head (6ba98b6) 89.12%. Report is 5 commits behind head on main.

:exclamation: Current head 6ba98b6 differs from pull request most recent head e726f78. Consider uploading reports for the commit e726f78 to get more accurate results

Files	Patch %	Lines
dtbase/core/db.py	58.62%	12 Missing :warning:
dtbase/core/utils.py	76.47%	4 Missing :warning:
dtbase/backend/api/sensor/routes.py	70.00%	3 Missing :warning:
dtbase/backend/api/model/routes.py	88.23%	2 Missing :warning:
dtbase/core/models.py	92.00%	2 Missing :warning:
dtbase/models/utils/backend_call.py	88.88%	2 Missing :warning:
dtbase/webapp/user.py	60.00%	2 Missing :warning:
dtbase/core/db_docker.py	66.66%	1 Missing :warning:
dtbase/models/utils/dataprocessor/get_data.py	92.85%	1 Missing :warning:
dtbase/tests/conftest.py	91.66%	1 Missing :warning:
... and 1 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #172 +/- ## ========================================== + Coverage 88.40% 89.12% +0.71% ========================================== Files 71 71 Lines 4864 4855 -9 ========================================== + Hits 4300 4327 +27 + Misses 564 528 -36 ```

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[mhauru]

Moved the login and auth_backend_call functions to core, as discussed on a call.