Unittests for SPDHG take long time

[paskino]

Unittests for SPDHG are done with the ASTRA cpu projector and on relatively large data, taking long time. We should reduce the amount of data that is processed.

https://github.com/TomographicImaging/CIL/blob/a2e45fe0aafe6329ec9027bda1cf262d4aff0284/Wrappers/Python/test/test_algorithms.py#L757-L770

[gfardell]

test_SPDHG_vs_PDHG_explicit and test_SPDHG_vs_PDHG_implicit are particularly slow. The assert_almost_equal on such a large array is probably one of the biggest costs.

[MargaretDuff]

Thanks Gemma - have added this comment to #1644 PR which attempted to fix the slow tests

[MargaretDuff]

Closed by #1898