Adds a simple type conversion to the meta file JSON

Hi @charlie.hoy. Recent changes to bilby added the waveform generator class to the result meta data. This then breaks the meta file writing to json.

I've added two commits here. The first just fixes the immediate problem: allowing the meta file to be written with bilby 0.6.1.

The second commit uses the bilby JSON encoder. This adds a lot of functionality and allows a number of things to be saved and loaded in a nice way. But, and perhaps more importantly, it means that changes to the bilby code won't break the pesummary workflow.

That it doesn't break the workflow is quite important: the successful upload of the bilby results relies on pesummary completing without raising an error.

If you really don't want to have import bilby feel free to take out the second commit. But, it would really speed up development if we didn't need to fix it in two places at once :)