add indirection for update step, add projection for LocationScale

[Red-Portal]

This PR adds a wrapper around Optimisers.update! so that we one can add additional post or pre-hoc operations. For instance, for LocationScale distributions, it is better to ensure that the diagonal of the scale matrix is always positive. This can be done by adding a projection after the call to Optimisers.update!.

[Red-Portal]

@torfjelde Any potential concerns/comments on this?

[Red-Portal]

@torfjelde Could you take a look when you have time?

[torfjelde]

Added a few comments:)

But worth noting that I'm very much out-of-the-loop with the current design so it's difficult for me to give good feedback beyond just the delta of the PR 😕

[Red-Portal]

@torfjelde Thanks for the review! Yeah, comments on the deltas would be much appreciated, especially when they're design-related.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.33%. Comparing base (dbdb13e) to head (538bafe).
Report is 14 commits behind head on master.

❗ Current head 538bafe differs from pull request most recent head b2b09bb

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
+ Coverage   91.93%   93.33%   +1.39%     
==========================================
  Files          11       11              
  Lines         186      195       +9     
==========================================
+ Hits          171      182      +11     
+ Misses         15       13       -2     

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