a couple of tiny optimizations

[ExpandingMan]

Hi. I was taking a look at this package as part of #7273 in so doing made these (very tiny) tweaks.

• Kerr metric parameters that depend on each other can now take them as keyword arguments. When I had done this, I expected these were used many times in the integrals, and would therefore be performance critical and in need of even this extremely tiny performance optimization. As it is they are not used much so the real world performance benefit here is miniscule.
• In CoordinatePoint(), there was a fallback that was not being hit in any of my tests where it could return a Vector{Float64} instead of a NTuple{4,Float64}. This would cost an allocation unless the compiler elides it, and could cause type instability if used improperly since the inferred return type here was a union.
• AbstractMaterial was declared twice (causing warnings from the docs tools). The one I did not remove had to stay where it was because of declaration ordering.
• Fixed a deprecation warning (from using ADAM SGD, now called Adam) in one of the examples.

Pretty trivial stuff, but figured all that would be uncontroversial so I might as well offer to merge my tweaks.

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.86%. Comparing base (c81da7c) to head (7ca8bf4).

Files with missing lines	Patch %	Lines
src/materials/CoordinatePoint.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #19   +/-   ##
=======================================
  Coverage   82.86%   82.86%           
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  Files          19       19           
  Lines        1640     1640           
=======================================
  Hits         1359     1359           
  Misses        281      281           

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