Provide control over the source of the timestamp

[ericonr]

ADVimba already supports this: https://github.com/areaDetector/ADVimba/blob/e2b9e4fc3d7df04c1b5ad8c50dc0d2d64e2a386e/vimbaApp/src/ADVimba.cpp#L611

An implementation was attempted by Emilio: #25 , but it wasn't merged. Maybe recovering that is enough?

An improvement could be to add a choice for #17, i.e. whether to fix the timestamp for EPICS epoch or not.