Add the apply function

[ibell]

Some more thinking is needed about what to do about padding out the order

[LBouck]

I think that probably implementing an adaptive Chebyshev approximation scheme would be best. The user just inputs the function and then we adaptively create the Chebyshev expansion. When we use apply, we do what we do in overloading multiply and resample on the chebyshev grid. Once we have an adaptive constructor, we could reapply the adaptive constructor on the new function.

[LBouck]

Let me know what you think.