Fix for using Firedrake with CUDA

nblock and the size of datasets seem to be `numpy.int64` types when running with Firedrake.

This commit converts them to `int` before passing them to PyCUDA. 

The Firedrake tests can pass with the CUDA backend, with the following exceptions:

* The identity test for degrees 3 and 4 is problematic because it makes cicc (part of the CUDA compiler) go into a slow loop consuming 100% CPU. It eventually finished compiling for degree 3 after about 10 minutes but I didn't wait for the degree 4 case to finish compiling. 
* The identity_parallel test fails - I think this would be expected anyway - IIUC this tests with MPI.

Thanks to @wence- for suggesting that this is the correct fix.

Buildbot pass: http://buildbot-ocean.ese.ic.ac.uk:8080/builders/pyop2-testing/builds/499