Also updated API version guard.

Step size of zero generates an invalid sampling grid.

Also:
- Removed unnecessary header includes from cpu/approx.cpp
- Minor c/approx.cpp cleanup
- Minor approx1/2 test cleanup

- Check for non-monotonic positions.
- Check for approx behavior with af::Inf values.

Also referencing snippets from test framework.

The approx CPU kernel implementation was calling the interp() kernel
with the incorrect dimension values. This was causing the
CPPUniform{Row,Column}MajorInterpolation tests to fail previously.
Also improved approx1 and approx2 tests.

All tests performing interpolation. Improved test names, removed
unnecessary tests, reordered tests