Automatically find requirements in setup.py, add new dev and example requirement files and update check_formatting action to use conda instead of pip

[w-k-jones]

setup.py now finds required packages by parsing requirements.txt, rather than having to maintain the list of packages in both

I've also added two new requirements files -- dev_requirements.txt and example_requirements.txt that contain the additional dependencies for developers and for running the example notebooks respectively.

The check_formatting action previously used pip to install requirements, however this broke when trying to install cartopy. I've changed it to use conda instead, in the same manner as the codecov-CI action

• Have you followed our guidelines in CONTRIBUTING.md?
• Have you self-reviewed your code and corrected any misspellings?
• Have you written documentation that is easy to understand?
• Have you written descriptive commit messages?
• Have you added NumPy docstrings for newly added functions?
• Have you formatted your code using black?
• If you have introduced a new functionality, have you added adequate unit tests?
• Have all tests passed in your local clone?
• If you have introduced a new functionality, have you added an example notebook?
• Have you kept your pull request small and limited so that it is easy to review?
• Have the newest changes from this branch been merged?

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (3bdc581) 54.60% compared to head (d125419) 54.60%.

Additional details and impacted files

@@            Coverage Diff             @@
##           RC_v1.5.0     #288   +/-   ##
==========================================
  Coverage      54.60%   54.60%           
==========================================
  Files             15       15           
  Lines           3170     3170           
==========================================
  Hits            1731     1731           
  Misses          1439     1439           

Flag	Coverage Δ
unittests	54.60% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tobac/analysis.py	8.48% <ø> (ø)
tobac/plotting.py	3.05% <ø> (ø)
tobac/wrapper.py	8.06% <ø> (ø)

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[JuliaKukulies]

All makes sense to me and looks good!