Fix cholesky.L and bugs related to copy

[TSGut]

Fixes #173

The issue arises because adaptive Cholesky factors can't be copied.

Here's a minimal example of the error before fixing:

z = 10_000;
A = BandedMatrix(0 => -2*(0:∞)/z, 1 => Ones(∞), -1 => Ones(∞));
M = Symmetric(I-A)

chol = cholesky(M)
copy(chol.factors') # fails as described in the issue

I will also add tests to verify chol.L and copy() don't silently break in the future.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.33%. Comparing base (b25d11f) to head (387a497).
Report is 9 commits behind head on master.

❗ Current head 387a497 differs from pull request most recent head 0ce1828. Consider uploading reports for the commit 0ce1828 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
+ Coverage   81.53%   84.33%   +2.79%     
==========================================
  Files          10       11       +1     
  Lines        1381     1417      +36     
==========================================
+ Hits         1126     1195      +69     
+ Misses        255      222      -33     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[TSGut]

think this is ready for review or merge @dlfivefifty

not sure what the pre-job thing is that CI is stuck on, otherwise I would merge it myself

[TSGut]

ok I think that addresses everything

[TSGut]

Good to merge?