Align DEMetropolis defaults with literature recommendations

[gbrunkhorst]

What is this PR about?
Changing the defaults for DEMetropolis and DEMetropolisZ samplers to proposal_dist=pm.NormalProposal and tune='scaling' as discussed with @michaelosthege here. This is for consistency with the approach in terBraak2008 paper (which discussed using a proposal distribution with unbounded support such as the Normal distribution).

The changes consist of two lines of code plus docstrings for each sampler.

Tests. I followed the instructions here and had to update the test_metropolis.py file to match the new defaults.

Major / Breaking Changes

• Since the default assumptions are changed code that uses defaults would be slightly different. However, this should be an improvement.
• Specifying the arguments tune and proposal_dist allows one to revert to the previous settings.

New features

• no

Bugfixes

• no

Documentation

• documentation has been updated consistent with modifications

Maintenance

• no

[michaelosthege]

Just adding a few small changes to fix pre-commit (black would complain about ' strings) and aligning the docstrings

[codecov]

Codecov Report

Merging #6488 (7bbd7f4) into main (902b1ec) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6488      +/-   ##
==========================================
- Coverage   94.79%   94.79%   -0.01%     
==========================================
  Files         148      148              
  Lines       27732    27734       +2     
==========================================
+ Hits        26289    26290       +1     
- Misses       1443     1444       +1     

Impacted Files	Coverage Δ
pymc/step_methods/metropolis.py	83.58% <100.00%> (-0.22%)	⬇️
pymc/tests/step_methods/test_metropolis.py	100.00% <100.00%> (ø)

[michaelosthege]

Thanks @gbrunkhorst for the thorough analysis in the other PR, and for updating the defaults here!