Dedalus hangs in parallel when attempting a 3D interpolation using a Chebyshev basis

If one attempts to perform an interpolation to a single point in 3D before the transposes are created, Dedalus will hang if one of the bases is Chebyshev.

A workaround is to force the transposes to be created before the interpolation, for example by doing

data['c']
data['g']

The attached script provides a minimal working example. It runs fine in serial, but hangs in parallel if the workaround remains commented out.

Other notes:

• the bug does not manifest if interpolation is done along only one dimension (even if it is the Chebyshev dimension)
• This problem occurs in practice if one wishes to drop a "probe" in the flow to get a field at a given point and output it to a high time-cadence analysis task. The simulation will hang if the analysis task is triggered before the first timestep (as it is by default)

Force a deterministic order to bases when interpolating over multiple bases. This fixes issue #39.

→ <<cset 3c591a74b7d2>>

Also, note that the workaround in the original post was completely spurious; it just happened that with only 2 cores, pretty frequently it would work anyway (because there were only two randomly chosen dict orders!)...