drop_dimension method for spaces

[mzargham]

proposing we add a method that can be applied to any space that will create a new space without one of its dimensions.

suppose

@space
class MySpace:
     a: int
     b: int

but then i did

MySpace = MySpace.drop_dimension('a')

the resulting space would be as if i had declared

@space
class MySpace:
     b: int