simplify / streamline data types

[maxheld83]

we currently have a lot of especially dataframes in qmethod, where arguably, matrices would do.

I'm not quite sure what the pros and cons are (I remember that one should take the "simplest" data type possible) for dataframes vs. matrices, but we might want to think about making this consistent.

It's not a dramatic issue per se, but it might make future developments and testing more robust and easier, maybe give a cleaner codebase.

[aiorazabala]

Related to #239

[aiorazabala]

Changed from data.frame to matrix:

• loadings,
• flagged,
• general characteristics:SD,

as per commit fa26315