create more test for functions

[skewballfox]

The initial test for mfcc only verified that the shape of the output was correct, but didn't test for any of the properties that a correct MFCC implementation would have.

We need to come up with a list of properties that an mfcc implementation would have, and create a set of test that verify that the output of our implementation possesses those properties.

a good place to start would be the wikipedia pages for mel-frequency cepstrum and the mel scale