deprecate method keyword

[mathause]

• [x] Closes #122
• [ ] Tests added
• [ ] Passes isort . && black . && flake8
• [ ] Fully documented, including whats-new.rst

This

• deprecates the method keyword from the mask* methods
• adds the backend setting in regionmask.set_options
• renames "rasterize" to "rasterio".

As always I am a bit unsure if this is a good idea. Maybe I am just so used to the way it is done. The reason I want to do is that (i) to hide the keyword (ii) the name method is misleading. The good thing is that this should barely affect anyone as there is no reason to set method="..." (except for testing and rare cases). So I'll mark this as draft and let it sit for some days to get used to it.

[codecov-commenter]

Codecov Report

Merging #385 (221656e) into main (c79e412) will decrease coverage by 0.48%. The diff coverage is 83.78%.

:exclamation: Current head 221656e differs from pull request most recent head 31abddd. Consider uploading reports for the commit 31abddd to get more accurate results

@@            Coverage Diff             @@
##             main     #385      +/-   ##
==========================================
- Coverage   98.73%   98.24%   -0.49%     
==========================================
  Files          16       17       +1     
  Lines        1423     1427       +4     
==========================================
- Hits         1405     1402       -3     
- Misses         18       25       +7     

Flag	Coverage Δ
bare-minimum	91.51% <83.78%> (+1.03%)	:arrow_up:
min-all-deps	97.48% <83.78%> (+0.94%)	:arrow_up:
shapely-2	?
unittests	98.24% <83.78%> (-0.49%)	:arrow_down:

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

Impacted Files	Coverage Δ
regionmask/core/options.py	78.57% <28.57%> (-12.34%)	:arrow_down:
regionmask/core/mask.py	99.68% <96.66%> (-0.32%)	:arrow_down:
regionmask/core/regions.py	97.53% <0.00%> (ø)
regionmask/__init__.py	85.71% <0.00%> (ø)

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