Phconvert

What's new

Some fields were renamed, namely:
- `laser_pulse_rate` -> `laser_repetition_rate`
- `alex_period_excitationN` -> `alex_excitation_periodN`

Also, always save _binary ASCII strings_ (instead of _unicode strings_) to maximize file compatibility.

What's new?

When saving Photon-HDF5 files with `phconvert`, the fields in `detectors_specs` are now casted to be the same type as `detectors` array.

This change does not affect users reading Photon-HDF5 files in Python and MATLAB, but it may simplify the life (and potentially remove a source of confusion) for users using strongly typed languages.