Add a PreconditionedDirection direction rule

[kellertuer]

This new gradient rule allows to easily precondition the gradient as (preconditioning by 0.5):

gradient_descent(M, f, grad_f, p0;
    direction=PreconditionedDirection((M, p, X) -> 0.5 .* X),
)

or similarly in-place

gradient_descent(M, f, grad_f, p0;
    direction = PreconditionedDirection(
        (M, Y, p, X) -> (Y .= 0.5 .* X);
        evaluation=InplaceEvaluation()
    ),
)

📋

• add this also as a preconditioner=keyword to quasi newton (conjugate_gradient accepts that as an input already)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.88%. Comparing base (dd64c48) to head (87f8d8c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #444   +/-   ##
=======================================
  Coverage   99.88%   99.88%           
=======================================
  Files          80       80           
  Lines        8541     8562   +21     
=======================================
+ Hits         8531     8552   +21     
  Misses         10       10           

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.