reverse arguments to atan2

This makes the order of the 2 args consistent with other maths libraries
including ruby's Math.atan2; ie
    GMP::F(y).atan2(x) == Math.atan2(y, x)
the macro is also used to implement agm and hypot, but for those
functions the order doesn't matter.
allow float argument to atan2
fix a compilation warning
Merge branch 'master' of https://github.com/timpeters/gmp
oops, should be expecting floats