Add erf function

[benjello]

This PR adds an erf function using scipy.

[gdementen]

I do not want to depend on scipy, so the function must be either optional (only available if scipy is installed) or computed via an expression like shown at http://stackoverflow.com/questions/457408/is-there-an-easily-available-implementation-of-erf-for-python, if the later approach gives sufficient precision.

[benjello]

@gdementen : what is your favorite implementation ?
I would choose an optional scipy as a standard practice but the function you gave may be faster
and sufficient for liam2 needs.

[gdementen]

I have no idea if what I linked to is precise enough. I am not really qualified to tell. I would accept either, so do what you prefer/think is best. The second option might be faster indeed thanks to numexpr/multi-threading, but that would need to be tested.

[gdementen]

I cannot find to code of the erf function in scipy, it seems like it links to a C implementation from some other lib, so it is unlikely we will get any faster, at least with a single thread.

[benjello]

There is a link to the implementation in C on this page.

I will use the scipy version.

[benjello]

If the implementation suits you I can even write a line in the changelog ;-)

[benjello]

@gdementen : any chance this get merged soon ?

[gdementen]

Sorry for the awful delay. It completely went off my radar. If you could add a note in the documentation & changelog, that would be great.

[gdementen]

I would like a better message for people trying to use it without scipy installed. Something like:
"Failed to import scipy, which is required for erf(). Please make sure scipy is installed and working."

[benjello]

Needs to be done:

• changelog
• note in documentation

[benjello]

@gdementen : rebased, doc and changelog added

[gdementen]

Thanks