(WIP) enzyme integration

[samuelpmishLLNL]

a preliminary example of integrating enzyme into serac as the primary mechanism of material model differentiation

Some notable benefits of enzyme over serac::dual:

• excessive use of auto in material models is not required
• qfunction arguments can also be regular C++ types, rather than auto
• bodies of qfunctions do not need to use the C++ type system for AD, so some programming constructs (conditionals, operator+=, etc) are much easier to write
• support for reverse mode differentiation

some notable downsides/changes include:

• enzyme is relatively new and has some sharp edges (e.g. currently, some qfunctions that use serac::zero are crashing enzyme).
• requires finite element kernels to be compiled with clang compiler. clang and enzyme do claim to support AMD/NVIDIA GPU backends, but I have not verified that yet.
• must compile with optimization or enzyme doesn't work (this is fixed if we adopt C++20)