pushForward of ToricCycle

[sameerav]

Given a closed embedding of varieties Y \subseteq X and a ToricCycle C on Y, pushForward(C,X) should return the corresponding ToricCycle on X.

In this case, the normal toric variety Y should have a "parent" attribute in the cache, and Y.parent should be X.

X is an optional parameter. If X is omitted, C is pushed forward to parent Y. Else, if an X is given, we proceed up the parent chain until we find parent \dots parent Y === X.