The CSC matrix API mirrors the CSR matrix API. However, there
are subtle differences throughout (both in the available
methods and the implementation) that I believe makes any attempt
to avoid the duplicate effort futile.
This mimics how std does it, e.g. std::vec::Vec. This avoids potential
problems down the road, where adding more types might clutter the
API interface and generated documentation.
SparsityPattern's constructor now returns a fine-grained error
enum that enumerates possible errors. We use this to build a more
user-friendly error when constructing CSR matrices.
We also overhauled the main SparseFormatError error type by
making it a struct containing a *Kind type and an underlying error
that contains the message.
