Set up rkeops options to use 64bit float precision in computation
when calling user-defined operators.
Details
By default, rkeops uses 32bit float precision in computation.
It is mandatory to call rkeops_use_float64() to enable 64bit float
precision in computation.
Important: 32bit float precision computations are faster than 64bit float, however the lower precision may have a huge effect on the accuracy of your computation and validity of your results in certain applications.