Separate sym factor

[luke-kiernan]

Changes:

1. Use multiple dispatch instead of @eval macros for figuring out which function to call among the real/complex and 32-bit/64-bit indexing combinations.
2. Add a full_factor argument to klu: by passing full_factor = false, you can delay doing the numeric factorization until later. I also added nonzeros, so that there is an interface that allows for setting the nonzero values after construction.
3. Export the functions that do symbolic/numeric factorization: klu_analyze!, klu_factor!. I also added klu_refactor! as an alias for klu!, for naming consistency with the SuiteSparse functions.

I couldn't easily figure out documentation/formatting stuff, so I did my best to match what you have there.

[codecov]

Codecov Report

Attention: Patch coverage is 66.36364% with 37 lines in your changes missing coverage. Please review.

Project coverage is 56.27%. Comparing base (7479676) to head (e9996f2).

Files with missing lines	Patch %	Lines
src/KLU.jl	68.05%	23 Missing ⚠️
src/type_resolution.jl	63.15%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
+ Coverage   55.24%   56.27%   +1.02%     
==========================================
  Files           2        3       +1     
  Lines         572      606      +34     
==========================================
+ Hits          316      341      +25     
- Misses        256      265       +9     

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