Move to a more pythonic API

[otizonaizit]

The naming of the functions in the public API are still derived by the MATLAB implementation. The MATLAB implementation does not use name spaces, I guess because it used to be the case that to make a function visible it needed to have the same of the file it is defined in. As a result we have to do something like:

import psignifit as ps
import psignifit.psigniplot as psp

res = ps.psignifit(...)
ax = psp.plot_psychometric_function(res)

for the function psignifit is indeed crazy, without the aliasing in __init__
we actually have psignifit.psignifit.psignifit.

I think that it makes more sense to name a function by what it does, so I think we want an API like this instead:

import psignifit as psf

res = psf.fit(...)
ax = psf.plot_psychometric_function(res)
ax = psf.plot_stimulus_residuals(res)
...

In other words this means we could have a flat namespace with functions that have names not as typo-inducing as psignifit is. Given that we do not have a lot of functions in the public API, I think a flat namespace is the easiest and more intuitive thing to do.

All of this could be done when we address #228 . Also, it can be done in a 100% backwards compatible way by creating aliases for the old API that can be supported forever.