Add better enum support for "experimenttype"

As a user of the API I would like to avoid having accidentally sent invalid data to ispyb. One place in particular where this is easy to go wrong is with enums as I may have an accidental typo in the string representation of it. e.g. it would be nicer to be able to do something like:
```python
from ispyb import open, ExperimentTypes

with open(config_file) as conn:
params = conn.mx_acquisition.get_data_collection_params()
params["experimenttype"] = ExperimentTypes.MESH
```