Added max(x,y) and abs(x) for vexcl vectors

I know fmax and fabs are already defined, but adding overloads named max and abs too was necessary for me to pass vexcl::vector to a template library that uses the cmath functions by these names.

This is an incomplete solution for abs (ideally there should be separate overloads for integral and floating point types, and only the latter should hand off to fabs in the OpenCL kernels) but it's at least a monotonic improvement in compatibility.