Update BaristaSeq text, fix LinearUnmixing

[ambrosejcarr]

This PR addresses scientific issues in BaristaSeq:

• LinearUnmixing was doing the wrong thing (summing over the wrong axis, which led to incorrect unmixing)
• Updates text in BaristaSeq.py
• BaristaSeq is small enough that it can be rendered by readthedocs as it stands, so testing it there and removing the out-of-date .py and .ipynb

If we decide to keep checking in notebooks, this can be reversed by downloading the .ipynb file and checking it back in.

[codecov-io]

Codecov Report

Merging #1188 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1188      +/-   ##
==========================================
+ Coverage   88.15%   88.16%   +<.01%     
==========================================
  Files         129      129              
  Lines        4949     4950       +1     
==========================================
+ Hits         4363     4364       +1     
  Misses        586      586

Impacted Files	Coverage Δ
starfish/image/_filter/linear_unmixing.py	96.87% <100%> (+0.1%)	⬆️

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[dganguli]

should un-delete the notebook for spacejam. pretty please?