Use pyproj TransformDirection enum for better performance

[djhoese]

As mentioned by @snowman2 in #532, using the pyproj enum for these operations is slightly faster than not using it. This is a simple import and usage.

• Closes #xxxx
• Tests added
• Tests passed
• Passes git diff origin/main **/*py | flake8 --diff
• Fully documented

[codecov]

Codecov Report

Merging #536 (93e8178) into main (046acf6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #536   +/-   ##
=======================================
  Coverage   94.26%   94.26%           
=======================================
  Files          82       82           
  Lines       13052    13055    +3     
=======================================
+ Hits        12303    12306    +3     
  Misses        749      749           

Flag	Coverage Δ
unittests	94.26% <100.00%> (+<0.01%)	⬆️

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

Files Changed	Coverage Δ
pyresample/_spatial_mp.py	83.82% <100.00%> (+0.11%)	⬆️
pyresample/geometry.py	87.65% <100.00%> (+<0.01%)	⬆️
pyresample/slicer.py	90.76% <100.00%> (+0.07%)	⬆️

[coveralls]

coverage: 93.838% (+0.002%) from 93.836% when pulling 93e8178 on djhoese:bugfix-proj-enums into 046acf6 on pytroll:main.