Hybrid pipeline

[nestordemeure]

Added hybrid GPU pipelines, able to run mixes of CPU and GPU operators.

• adds a SetDict class to utils.py to easily perform set operation on lists of keys that might be on GPU
• update data movement infrastructure in Pipeline class
• add a TOAST_GPU_HYBRID_PIPELINES environment variable (defaults to True, can be set manually to False) to forbid hybrid pipelines (one can also forbid them locally by setting a class variable)
• update the requires / provides of several operators (most of them untested until now)

[nestordemeure]

The best formulation I can come up with is this (in both cases things might already exist, or not):

requires == "Things which might exist and will be read",
provides == "Things which might exist and will be modified".

reads and modifies maybe?

[tskisner]

Something for a future PR...