[WIP] Add a default time value

[adrn]

I think most potential methods should have a default time value so that you can call, e.g.,

pot.gradient(xyz)

without having to specify a time.

I marked this as a WIP because I'm not sure this is the best path, but here is one possible implementation. I added new dispatches with default values and added a global definition of the default time value (0 - does any other time make sense??).

A few things to consider / still need to be done:

• Should this only be allowed on static potentials? If so, we need to add some easily JIT-able way of detecting whether a potential is static or has time-dependent parameters.
• The default time is currently a Quantity, but it should really be a dimensionless value. The only default times I think are worth discussing are 0, inf, -inf, or nan. Probably only 0 makes sense. None of them need to have a unit attached.

[nstarman]

I really like this. I wonder if this isn't related to the discussion about having xyz work with a scalar r if it's spherically symmetric... We discussed some kind of MD or flag or something. I recall there being difficulties with optimizing this for JIT...