Changes

Moved the ghost-point setting of xm(1) = xm(2) to the last line
of the subroutine so this command doesn't get overwritten.

This is technically BIT_CHANGING because it alters rtm (and some
rtm-related calculations) at the ghost point, but it is not
answer changing.

This is being done in preparation for ghost point removal.

Optional arguments are just best sometimes (#1150)

* Adding optional argument to zm2zt/zt2zm to act as threshold. This cuts down on lots of bulk and reduces the chances of breaking the GPU code. All BFB.

* Removing TODO

* Replacing hardcoded 0.0_core_rknd with zero_threshold