Add reuse flags and Cholesky or LU structs for direct solvers

[sean-engelstad]

• add a reuse_factorization flag in direct_cholesky and direct_LU flags
• make a class or some data structure for reusing the cholesky and LU data
• demonstrate that this can solve multiple RHS effectively
• maybe a reuse object or struct can be passed out of it, and if nullptr input then it isn't in reuse? That may be cleaner than making a whole class for this. Would hold data like info_L, descr_L, pBuffer, etc.