Change FieldTimeSeries to behave like 1D vector?

[glwagner]

This really depends where we want to take this abstraction.

One possibility is to develop "four-dimensional fields". This is sort of what the design reflects now (ie we subtype AbstractField, etc).

Another possibility is to ditch that and cement Fields as 3 dimensional and less, and commit the FieldTimeSeries abstraction to look like a 1D vector of 3D fields.

It seems like a subtle distinction, but is has some important consequences. For example, what should

first(fts::FieldTimeSeries)

return? If FieldTimeSeries is 4D then this is either parent(fts)[1, 1, 1, 1] or fts.data[1, 1, 1 ,1]. But if FieldTimeSeries is 1D this is fts[1] --- the first Field. I think it's nice to have first(fts) and last(fts) return fields --- we use stuff like that a lot, but right now we are forced to write stuff like

Nt = length(fts.times)
fts[Nt]

kind of annoying.

Right now the API is hybrid; ie we have getindex(fts, i) and getindex(fts, i, j, k, n). But we use the "vector" abstraction more than the "4D field mode" a lot more I think.

One downside is that this might preclude defining a ∂t abstract operation.

I don't think this impacts support for reductions, but I have to think about that.

[glwagner]

Done