Parameter specific learning rates for Gradient Descent

[BradyPlanden]

Description

Adds per-parameter learning rate to gradient descent. This is achieve by creating our own implementation of gradient descent to replace the Pints' composed one.

Issue reference

Fixes #567

Review

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.15%. Comparing base (c8b00e6) to head (ea2c6e7).
Report is 102 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #569      +/-   ##
===========================================
+ Coverage    99.08%   99.15%   +0.06%     
===========================================
  Files           52       56       +4     
  Lines         3605     3893     +288     
===========================================
+ Hits          3572     3860     +288     
  Misses          33       33              

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