GPU support for Cartesian problems

[kburns]

This PR adds GPU support for one dimensional bases and cartesian problems. Remaining rough edges include good defaults for subproblem coupling, and raising or warning for unsupported features (GPU+MPI, GPU+curvilinear).

[csskene]

Just noting here that the trace operator doesn't work on GPU's as cupy's einsum doesn't take an out argument. A similar fix to dot product (commit fb9b3d6) fixes things.
Also problem.add_equation will not work for float32 as unify attributes for dtype raises an 'Objects are not all equal' error.

[csskene]

Interpolate doesn't currently work either, with error
...
File "~/Packages/dedalus_gpu/dedalus/core/operators.py", line 975, in _subspace_matrix
return cls._full_matrix(input_basis, output_basis, *args)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: InterpolateRealFourier._full_matrix() missing 1 required positional argument: 'position'