Remove regression

I propose to remove the `regression` keyword from functions, mainly `anafast` and `smoothing`.

`regression` is a boolean option, if `True`, the monopole of the map is removed before performing spherical harmonic transforms.
The problem is that due to some bugs in the past, the default behaviour of `healpy` changed, see #167. Before it had `regression=True` but was not actually removing the monopole due to a bug. Now `regression` is still `True` but it is now actually removing the monopole.

At this point I think the most sensible option is just to remove this keyword so that users have to run `remove_monopole` explicitely before `anafast` or `smoothing`.

Existing scripts with `regression` keyword set will fail.
Existing scripts with `regression` not set will restore the default behaviour of `healpy` ~1.6 and not remove the monopole.

Any though on this?