PSI detector base

Problem

Review usage of filepath for sending a file event message to BEC.

https://gitlab.psi.ch/bec/ophyd_devices/-/blob/main/ophyd_devices/interfaces/base_classes/psi_detector_base.py?ref_type=heads#L114

Possible solution

Make filepath a signal in psi_detector_base. Check if this corrupt existing signals of area detector to avoid using the same or a similar namespace.