ftarget is missing from the C interface

[zaikunzhang]

Hi @jschueller ,

ftarget is an essential input from the user. I think it will be good to include it in the C interface.

If the user does not want to / cannot specify ftarget, then he/she should input -INFINITY, where INFINITY is the micro in math.h for representing infinity. I hope my understanding is correct and I hope INFINITY is standard C. It is a pity that C does not support optional arguments.

This was related to a previous comment from Tom on the Python interface, where the same comment applies, and we can default ftarget to $-\infty$ if there is no input from the user.

Thank you.

Zaikun

[zaikunzhang]

Closed by #63