Saving and loading fitted nomials

[tonystao]

There should be some way of saving/loading/manipulating nomials because it can take a long time to generate them.
During loads, it would be convenient to be able to rename variables.

(Pickling worked in the old implementation of nomials, but it doesn't work anymore.)

[whoburg]

👍 good issue

re: renaming variables, that's best handled via gpkit's substitution, as in

In [1]: from gpkit import Variable

In [2]: x = Variable("x")

In [3]: p = 1 + x + x**3

In [4]: p.sub({"x": Variable("y_{tony}")})
Out[4]: gpkit.Posynomial(1 + y_{tony} + y_{tony}**3)