Added some unit test and removed some print()

Closes #33 (closed) This merge request adds most of the missing unit tests. The remaining test uncovered code is either something super small, almost impossible to reach while doing unit test or a part of code that doesn't profit from a unit test (mostly because they're not a "unit")

Here is the last coverage report:

Name                     Stmts   Miss  Cover   Missing
------------------------------------------------------
app.py                      27      5    81%   18-21, 32-33
auth.py                     23     11    52%   11-29, 34
config.py                   38      6    84%   13, 18-22
extensions.py                4      0   100%
models/__init__.py           3      0   100%
models/base.py              22      6    73%   10-12, 40-42
models/data.py              12      1    92%   23
models/data_schema.py       11      1    91%   21
schemas/__init__.py          2      0   100%
schemas/data.py              7      0   100%
schemas/data_schema.py       6      0   100%
tests/conftest.py           81      0   100%
tests/test_data.py         127      1    99%   8
tests/test_utils.py         91      2    98%   275-276
views/__init__.py            0      0   100%
views/data.py              179      5    97%   333-337
views/utils.py              42      0   100%
------------------------------------------------------
TOTAL                      675     38    94%