Attempt reading old-style Matlab class in v7.3 as struct

[Jeroen-van-der-Meer]

If you save an old-style Matlab object (i.e. created in an @ClassName folder, before classdef was a thing) in a v7.3 file, MAT.jl throws an error saying it cannot read from a non-struct group (cf. line 232 of MAT_HDF5.jl). As a matter of fact, it appears that Matlab saves these objects 'as if' they are structs, and indeed the objects that I have appear to be read just fine if I manually remove the error and let the code continue.

Instead of throwing an error, I would propose giving a warning, such as

@warn "Unknown non-struct group of type $mattype detected; attempting to read as struct"

in case it can, in fact, be read properly.

[ViralBShah]

@Jeroen-van-der-Meer Would it be useful for you to have commit access to this package, so that merges don't have to wait this long? I just happened to notice this PR and it seemed straightforward to merge.

[musm]

I think we just forgot about it