Double precision ValueError not raised

[bb515]

A ValueError not raised if an opencl context is not provided and the context obtained from context = get_context() does not support double floating-point precision. Correct behaviour is to raise the ValueError.

https://github.com/alan-turing-institute/PeriPy/blob/master/peripy/integrators.py#:~:text=%23%20Get%20an%20OpenCL,floating-point%20precision%22)