Be compatible with both actx.np.acos and actx.np.arccos

Currently, neither acos nor arccos works (the former is not a member of _numpy_math_functions, while loopy does not know how to compile arccos). This MR fixes that.

Ugh, we've duplicated some work here: https://github.com/inducer/meshmode/pull/89

Closing in favor of mine. Sorry about the extra effort!

closed

Not problem. The alternative is apparently better. I was just trying to get pytential CI to work :P