The namespace declaration for googlehash is a configurable macro that
can be disabled. In particular, it is disabled within google, causing
compile errors since `dense_hash_map`/`sparse_hash_map` are then in
the global namespace instead of in `::google`.
Here we play a bit of gynastics to allow for both `google::*_hash_map`
and `*_hash_map`, while limiting namespace polution. Symbols within
the `::google` namespace are imported into `Eigen::google`.
We also remove checks based on `_SPARSE_HASH_MAP_H_`, as this is
fragile, and instead require `EIGEN_GOOGLEHASH_SUPPORT` to be
defined.
* change Sparse* specialization's signatures from <..., int Backend> to <..., typename Backend>. Update SparseExtra accordingly to use structs instead of the SparseBackend enum.
* add SparseLDLT Cholmod specialization
* for Cholmod and UmfPack, SparseLU, SparseLLT and SparseLDLT now use ei_solve_retval and have the new solve() method (to be closer to the 3.0 API).
* fix doc
