Rename tof coordinate to toa

[nvaytet]

The time-of-arrival of a neutron at the detector is commonly named time-of-flight or tof.

This name is however misleading because it is not necessarily the time the neutron spent in-flight,
since that also depends on the birth time of the neutron.

Only the arrival time is known at a neutron beamline,
and there is thus an inherent uncertainty in the birth time as neutrons can originate from any part of the pulse.
This is especially true at long-pulse neutron sources such as ESS.

We rename tof to toa in this PR to remove confusion around the definition of time-of-flight.
We keep the tofs properties and tof coordinate, and add deprecation warnings when they are accessed.

[jokasimr]

Looks good to me!