Superfluous error when parsing old-style class object in v7.3

[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"

cf. also associated PR

[musm]

closed by #198