Optional Variables

This MR adds optional variables to the variable registration.

Description

• Optional Variables require to be typed
• However, when unspecified, a optional variable is None

Things done in this MR

• added optional variables to mf

Check all before creating this PR:

• Documentation adapted
• unit tests adapted / created

Example Usage

def register(mf):
    mf.register_defaults('somevariable', mf.optional(int))

This call allows the user to optionally set --somevariable 42.
Not setting the variable sets its value to None.