Task/weak numba dependency

[johannvk]

This PR adds numba as a weak / optional dependency of skchange, instead of the hard dependency it is now.
To achieve this, we use the sktime function _check_soft_dependencies, and return a passthrough version of the @jit and @njit decorators if numba is not installed.

Additionally, this PR enables the configuring of some numba default @njit parameters through the setting of environment variables.
Currently the supported parameters are cache, parallel, and fastmath, set by the corresponding environment variables NUMBA_CACHE, NUMBA_PARALLEL, and NUMBA_FASTMATH.
The current implementation only sets the defaults, so if a user of the numba decorator specifies e.g. fastmath=False, their choice will not be overridden.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.98%. Comparing base (b66f3f4) to head (1e3f6fe).
Report is 34 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
- Coverage   97.06%   96.98%   -0.08%     
==========================================
  Files          44       33      -11     
  Lines        1873     1395     -478     
==========================================
- Hits         1818     1353     -465     
+ Misses         55       42      -13     

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[Tveten]

Another thing: I see the patch coverage is pretty low. Do you have time to write a few tests for the new njit module @johannvk? They can be placed in skchange/utils/numba/tests

[johannvk]

Another thing: I see the patch coverage is pretty low. Do you have time to write a few tests for the new njit module @johannvk? They can be placed in skchange/utils/numba/tests

It took some trial and error, but I added tests for all the new configuration fuctionality now, and also added another test step to the Github Actions workflow to run all our tests with and without Numba jit-compilation.