bugfix and almxfl

Created by: zonca

• see 07da5038 for the bug we were discussing via email.
• I exposed some more of the libsharp objects properties to Python
• I also implemented almxfl functionality I needed in TOAST.

Please send me feedback on almxfl, I have implemented it in cython, I wonder if @mreineck would prefer to port to C and then we can call it from Python instead. Or if it is fine to have it there.