feat: skip fields with undocumented classes instead of hard error

[mcmcgrath13]

Currently, if a .mat file contains fields such as audioplayer and others that are undocumented, matread will error without returning any read data. This pull request adds functionality to warn the user and return missing instead. The functionality is implemented similarly in both MAT_V5 and MAT_HDF5, the main difference I've noted is that these undocumented fields sometimes come with a ghost field at the root level which is handled slightly differently.

In my work, I am trying to process .mat files with many fields/structures/sub-structures and often these undocumented fields are not the ones I am trying to work with and it is frustrating when the read process crashes (and in V5 I can't work around this by reading in only sub-paths of the file). This PR will allow working with the rest of the file and skip only what is unreadable.

This is related to #23 , but instead takes the approach of skipping these fields. If #23 is ever completed/incorporated, this should become obsolete functionality.

[mcmcgrath13]

This may actually be redundant to #118 for v6/v7 files. It reads in the data, it doesn't make any sense to me based on what I see in MatLab, but it doesn't crash, so that works for me.

The HDF5 may still need this to not fatally error on these classes.

[mcmcgrath13]

@yuyichao @timholy how would you like to proceed with this and #118? Thanks!

[timholy]

Merged #118. Thanks though!