ENH: Better named functions

[HaoZeke]

consider increased consistency of function names? e.g fmm_to_sparseMatrix vs sparse_to_fmm. sparse_to_fmm was a bit confusing to me in that if I understand correctly, the package is 'fast', not the target format itself (?). matrix and vector is abbreviated to spare a few characters, while sparse_matrix is not. A vague suggestion: possibly follow the pattern of fmm_write_vector, fmm_read_to_vector for all formats?

First noted here: ropensci/software-review#606 (comment).

This is a breaking change, so I'm not too sure, but then again, better now than any other time. I'm OK with fmm myself, since its the package name, but yeah consistency is a good target.

[HaoZeke]

Also, there are some minor differences between Matrix Market files and what fast matrix market supports, including higher than single precision digits. Since the package name is abbreviated to fmm (in C++). fmm_to_sparse_Matrix is different because it constructs the sparse matrix as implemented in Matrix, a dgCMatrix.

sparse_to_fmm has been converted to sparse_Matrix_to_fmm