ci: modified test suite to run on cupy

[mrava87]

This PR modifies the entire test suite to run tests both using the numpy and cupy backends.

An environment variable called TEST_CUPY_PYLOPS is used to select whether to run the tests with the numpy (0) or the cupy (1) backends.

Basically one can run tests with the following commands:

# numpy
export CUPY_PYLOPS=0; export TEST_CUPY_PYLOPS=0; pytest
# cupy
export CUPY_PYLOPS=1; export TEST_CUPY_PYLOPS=1; pytest

where the first two exports aren't really needed if cupy isn't installed.

Additional requirements and environment files are provided to create cupy-enable Python environments for the CI.

[mrava87]

@cako as I previously mentioned, with the help of @hongyx11 we are trying to create a self-hosted CI to test our cupy backend for the operators and solvers. Since this CI would require access to a GPU, Yuxi has kindly offered to host in on Perlmutter using a similar setup that he has in place for another project.

Would you have time to have a quick look at this PR; it is pretty boring as most code changes are as explained above to allow switching between numpy and cupy via an environment variable. However, whilst going through this refractoring of the tests, I also found a few small bugs #657 #658 #659 (always good to fix things...)

@hongyx11, I guess we need this merged before we can try together to set up the self-hosted runner, right?