Update fftfilt implementation to mirror Matlab's

New implementation has the same behavior as Matlab's.
Update docstring specifying the level is 0 dB

Although the code uses a reference level of 0 dB for a signal with an
RMS of 1, the docstring still said the reference was 100 dB.
Add optional `normalize` parameter for wav_write
Create output directory if it doesn't exist

Checks if the output directory exists before writing the output data.
I'm not sure how good an idea this is, but it seems safe to me to check
first. Otherwise, the default is to try to write the data to the current
directory. It is not a bad idea, but it can rapidly create a mess.
Add optional models parameters to pred_to_pc

I added an optional parameter to the speech/experiment.pred_to_pc
function to specify the model to transform. Without this parameter, all
the rows were transformed using the same function, which is not really
useful.

The inputs can be a single model name, a list of model names, or a
dictionary with model names and output names. It think it offers the
most flexibility.

Another option would be to accept models directly, and then check their
`name` attribute. That would make things more straight forward, if the
users already have the models created, but it would be a bit awkward if
they are doing transforms from text file they just loaded...
I don't think it would hurt to add it...
Save speech material name in dataframe
Fix PEP8 errors
Merge branch 'feature/experiment' into develop
Remove tests from root directory

It came back due to a bad merge…
Merge branch 'hotfix/remove-root-tests' into develop

Conflicts:
	tests/test_mrsepsm.py
Set path to speech material and SSN directly

The main reason is that this does not break auto-complete; it is
possible to auto-complete the path to the SSN and sentences. The added
benefit is that the SSN does not have to be in the same root folder as
the sentence; it can be anywhere.