Add inline files

[boulderdaze]

Solver class has numerical implementation details that could be separated from the interface which makes the header files a little easier to read. There are two files that I worked on as a demo to discuss whether to have inl (inline) files separated from its header files.

[mattldawson]

I like this. It does seem easier to read with the function definitions in a separate file, kind of like normal .cpp files but for templated classes/functions. Would it make sense to put these in a separate root folder, like inl/ or inline/, or maybe just in src/?

[boulderdaze]

The projects I've seen that maintain header-only library with additional inline files seem to put them in the same directory but I think we could put them in the separate directory such as /inl, or inline. One thing I've noticed that most of micm don't seem to need to separate header file from inline functions. It may only be applicable to a few with math-heavy header files.

[mattldawson]

That sounds good to me - let's keep the .inl files in the include/ folder if that's most commonly done.

[codecov-commenter]

Codecov Report

Patch coverage: 97.77% and no project coverage change.

Comparison is base (de9c3eb) 97.19% compared to head (ba5cd27) 97.19%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #206   +/-   ##
=======================================
  Coverage   97.19%   97.19%           
=======================================
  Files          28       29    +1     
  Lines        2427     2428    +1     
=======================================
+ Hits         2359     2360    +1     
  Misses         68       68           

Files Changed	Coverage Δ
include/micm/solver/linear_solver.hpp	100.00% <ø> (ø)
include/micm/solver/state.inl	96.42% <96.42%> (ø)
include/micm/solver/rosenbrock.inl	96.65% <96.65%> (ø)
include/micm/solver/linear_solver.inl	100.00% <100.00%> (ø)
include/micm/solver/lu_decomposition.inl	100.00% <100.00%> (ø)

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