Implement Align (fix #79)

[thchr]

Here's an implementation of the covariance approach suggested in #79, focusing just on 2D.

I'm open to suggestions for how it could be extended to 3D in a general way - but I couldn't immediately imagine a nice and unified interface (we would need to align to two axes in 3D).

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 19 lines in your changes missing coverage. Please review.

Project coverage is 94.29%. Comparing base (c9e417a) to head (aabbbbb).
Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
src/align.jl	0.00%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
- Coverage   97.44%   94.29%   -3.16%     
==========================================
  Files           9       10       +1     
  Lines         549      578      +29     
==========================================
+ Hits          535      545      +10     
- Misses         14       33      +19     

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[hexaeder]

I invited you to the repo. Do you mind recreating the PR from an internal branch rather than your fork? That way we get the doc preview.

[thchr]

Superseded by #82.