reduce dependencies

[aaronspring]

Description

reduce dependency packages:
requirements.txt will only contain important packages for installing and running QuickStart
Not core functionality such as esmf for regridding, eofs for rel entropy, esmtools for stats will have to be installed by the user, won’t be in requirements but will be in environments and hence tested, but try import

Closes #454, #526

To-Do List

Make

• esmtools in stats.py, esmtools still needed for notebooks
• faster rm_poly than esmtools, from how to remove polynomial fit over cftime dimension? pydata/xarray#5016
• eofs for rel_ent.py
• Same for xrft

try install as xesmf is already. Then close. Add both in testing envs. Also strip those

[codecov]

Codecov Report

Merging #572 (58d9fcb) into master (ae343e2) will decrease coverage by 0.36%.
The diff coverage is 90.95%.

@@            Coverage Diff             @@
##           master     #572      +/-   ##
==========================================
- Coverage   94.40%   94.04%   -0.37%     
==========================================
  Files          54       54              
  Lines        5079     5022      -57     
==========================================
- Hits         4795     4723      -72     
- Misses        284      299      +15     

Impacted Files	Coverage Δ
climpred/comparisons.py	99.04% <ø> (ø)
climpred/horizon.py	96.55% <ø> (ø)
climpred/smoothing.py	87.65% <ø> (ø)
climpred/tests/test_compute_dims.py	100.00% <ø> (ø)
climpred/tests/test_perfect_model_prediction.py	100.00% <ø> (ø)
climpred/tests/test_utils.py	98.68% <ø> (-0.21%)	⬇️
climpred/relative_entropy.py	80.73% <50.00%> (-1.96%)	⬇️
climpred/stats.py	85.05% <69.69%> (-8.17%)	⬇️
climpred/tests/test_relative_entropy.py	92.30% <77.77%> (-7.70%)	⬇️
climpred/checks.py	95.23% <81.81%> (-1.61%)	⬇️
... and 22 more

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