CurPosition() returns the live axis position, which in this
implementation is inherently expensive to compute.
This shouldn't be required for the MMU, but it /will/ come in handy to
check for the axis position/s in Motion tests.
Since scheduling a move on a block which is being executed will jolt the
motors, be extra-safe and perform an extra lower-level check before
committing even if the caller is responsible.
Return the status, which can be useful to build a simple busy loop.
Yuri D'Elia