Add Capability to Load Models from CSV Files

[ColtAllen]

Models can presently be saved in either JSON or CSV format, but whenever attempting to load from CSV files I encounter the following error:

xarray.core.variable.MissingDimensionsError: 'chain' has more than 1-dimension and the same name as one of its dimensions ('chain', 'draw'). xarray disallows such variables because they conflict with the coordinates used to label dimensions.

I would appreciate assistance on this from anyone who is familiar with the arviz or xarray libraries, as this issue could probably be fixed in 1 or 2 lines of code.

[ColtAllen]

JSON files should be sufficient, so unless I get a request for persisting models in CSV format, I won't be working on this feature.

In v0.1b3 I'll append _json to BaseModel.save() and BaseModel.load() for clarification, after which this issue will be closed.