Phconvert

What's new?
- Fix a bug causing file validation to fail (fd0a278679e6d33ee15c9acc5ff194c03e6f36b0)

What's new?
- **Rewrite of validation function**: now they validates a HDF5 file, not an in-memory representation. This makes easier to test if a file is Photon-HDF5 compliant, regardless if it has been created by phconvert or not.
- Added more tests during validation
- Fixed error that caused authors and other identity fields not being saved

What's new?

A few important changes:
- Automatically convert input of "scalar" fields to scalar if they are arrays or sequences of length == 1. See 5.
- Check conformance with the specs in `save_photon_hdf5()` removing the need to call `assert_valid_photon_hdf5()` separately.
- Remove `time_reversed` field in `nanotimes_specs`. Now we use the simpler convention that `nanotimes` has a time axis direction correctly oriented. See 6.

What's new?
- Updated the descriptions (`TITLE` attributes) for various fields.

What's new?
- Try to address h5labview issue with empty strings by saving a single-space string in empty descriptions. See 4.

What's new?

Backward-incompatible changes:
- Change `dye_names` from array of string to a single string containing CSV text (8421904505fd12114d3877c6f6c965d8703e7862). Closes 2.

Other changes:
- Docs improvements