Unify CPU/GPU quadrature code

[janden]

From #692:

unify GPU to use CPU gaussquad source? (part of broader task to unify GPU with CPU sources where possible - but not much of common.cu is possible because has own CUDA stuff).

[ahbarnett]

Main tasks: have GPU code compile src/finufft_utils.cpp or rename as common_utils and use ifdefs to switch out any parts that the cpu or gpu should not compile.

Eg: gaussquad and next235beven should be moved from src/cuda/utils.cpp to the common utils.
This would leave src/cuda/utils.cpp empty, so we should kill it.

Eg: PI is defined in include/finufft/finufft_core.h but M_PI is defined in include/cufinufft/utils.h
Should rationalize.

Alex can sort out the makefile once CMake is working.