Print clear error if a user is tryng to load a .mat file version < 7.3

[traversaro]

Probably we can check if the .mat file is supported by looking at the starting bytes of the file, see:

def is_hdf5(file_path):
    with open(file_path, 'rb') as f:
        signature = f.read(8)
    return signature == b'\x89HDF\r\n\x1a\n'

Based on https://docs.hdfgroup.org/hdf5/v1_14/_f_m_t3.html#Superblock, it seems that this indeed a way to quickly check if a given file is hdf5 .