Synchronous Time for Field Maps in an `element`

Conceptually, there is an issue for a field map that varies with time having a synchronous time associated with it. Currently, this isn’t handled. If we use a element beam line element with a field map that varies with time it will be in global time and not local time and it will be replaced (not built unique in the component factory) so all instances in the beam line will have the same zero phase or synchronous time. We typically haven’t used field maps that vary with time in earnest so we haven’t noticed this.

BDSElement will need to have a flag of whether it is time dependent and therefore should be prepared uniquely for each use in the sequence (also geometry reloaded for unique logical volumes).
The same field definition may be used for multiple elements so the synchronous time must be associated with a copy of the BDSFieldInfo instance.

A test would be to create a simple field map that varies with t, e.g. an accelerating E field, and reuse it N times in a line. the particle should be accelerated N times. If it doesn’t work, it will de-phase and be decelerated.

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