pullback -- make the pullback along a toric map

Description

A toric map induces pullback operations in several different situations. Those currently implemented in this package are listed below.

pullback(ToricMap,CoherentSheaf) -- make the pullback of a coherent sheaf under a toric map
"pullback(ToricMap,Module)" -- see pullback(ToricMap,CoherentSheaf) -- make the pullback of a coherent sheaf under a toric map
pullback(ToricMap,ToricDivisor) -- make the pullback of a Cartier divisor under a toric map