More flexible import for matrix market format

[Andlon]

Currently the import of matrices to the matrix market format (provided by #1017) is unable to import, for example, an integer matrix into a CooMatrix<f64>. For usability reasons it would be good to make the import more flexible in this regard. Specifically, if the output format can store the matrix exactly (with no round-off or similar) then we should allow the conversion.

This however does not appear easy to implement, see some of the discussion in the comments of #1017.