Add support for various matrices in the MWFN loader

[tovrstra]

The MWFN file may also contain sections with integrals, which are currently not loaded yet.

Related to this, the loader assumes a specific order of sections in the MWFN file (the same order used by the MWFN code). This is probably fine, but makes it harder to implement optional sections (e.g. integrals). This can be addressed at the same time.

For more details, see discussion: #198 (comment)