Add volume function

[vihdzp]

To compute the volume of a polytope, we need to first check for planarity, then build an oriented simplex for each flag and add up all of their areas. The part of the oriented simplices might be made easier if we first made some sort of iterator object over flags of a polytope (which might then be reusable for other functions).

[vihdzp]

.volume() has been added, and will continue to be improved in the future.