Expose functions for validation

[pbotros]

From #3:

Another question is that could you please provide some interface like '_assert_decomp_successful' at https://github.com/carmenalab/emgdecomp/blob/master/emgdecomp/tests/test_decomposition.py#L140 for validation?

cc @shihan-ma

[pbotros]

It's a good point to expose! I bet people will often do their own validations with their own simulated data. I'll look into it, but in the meantime, copy/pasting the contents of that _assert_decomp_successful function into your own code is an acceptable hack.