Derivatives of empty arrays?

[Jacob-Stevens-Haas]

From differentiation.dxdt:

An empty X results in an empty derivative.

In the interest of ZOP #10, Errors should never pass silently, should dxdt() instead raise an error? It feels like returning an empty array instead of failing is an attempt to be cleverly helpful, but beyond the remit of these functions. Callers IMO should be responsible for the

if x.size:
  x_dot = dxdt(...)

Noticed when delving into an error in Derivative.x() in #15. I was refactoring the shape manipulation to be used by both x() and d(), and it gets (minorly) more verbose when handling empty arrays.

[andgoldschmidt]

I agree. Raising an exception is better, and I'm even more convinced because this created unnecessary verbosity in #15. Different implementations will have a different understanding of what sizes are too small (basic example is just increasing the order of finite differences), but there's enough of lower edge case when x.size is 1. We can eliminate the empty array while forcing the implementations to handle sizes starting at 1.