Tiler fix and preprocessing features

[dphogan]

Description

This pull request fixes a longstanding tiler bug where pixels beyond the edge of the source imagery have indeterminate value. This request also adds some features to the preprocessing library: Option to set the size of ShowImage output and add accompanying caption An "optical" module, currently with classes for converting color spaces *Reverting the multiprocessing command

Type of change

Please delete options that are not relevant.

• [X] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected - these changes will not be merged until major releases!)

How Has This Been Tested?

Checklist:

• [ ] My PR has a descriptive title
• [ ] My code follows PEP8
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new errors
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] My PR passes Travis CI tests
• [ ] My PR does not reduce coverage in Codecov

If your PR does not fulfill all of the requirements in the checklist above, that's OK! Just prepend [WIP] to the PR title until they are all satisfied. If you need help, @-mention a maintainer and/or add the Status: Help Needed label.

[codecov[bot]]

Codecov Report

Merging #383 into dev will decrease coverage by 1.62%. The diff coverage is 25.92%.

@@            Coverage Diff             @@
##              dev     #383      +/-   ##
==========================================
- Coverage   57.98%   56.35%   -1.63%     
==========================================
  Files          79       81       +2     
  Lines        6603     6911     +308     
==========================================
+ Hits         3829     3895      +66     
- Misses       2774     3016     +242     

Impacted Files	Coverage Δ
solaris/preproc/image.py	22.15% <0.00%> (-0.90%)	:arrow_down:
solaris/preproc/pipesegment.py	22.58% <0.00%> (ø)
solaris/preproc/optical.py	28.81% <28.81%> (ø)
solaris/tile/raster_tile.py	67.61% <60.00%> (-0.35%)	:arrow_down:
solaris/preproc/__init__.py	100.00% <100.00%> (ø)
solaris/eval/vector.py	85.88% <0.00%> (-7.74%)	:arrow_down:
solaris/nets/transform.py	62.09% <0.00%> (-0.81%)	:arrow_down:
solaris/nets/model_io.py	17.72% <0.00%> (-0.47%)	:arrow_down:
solaris/nets/train.py	12.04% <0.00%> (-0.30%)	:arrow_down:
solaris/nets/zoo/__init__.py	100.00% <0.00%> (ø)
... and 3 more

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