Solution file metadata not preserving tuples for multiple carriers when saved to NetCDF

[brynpickering]

If there are multiple carriers in or out for a technology (e.g. for CHP in urban scale model, both 'power' and 'heat' are carriers out) then a tuple of those carriers is given in the model.solution.metadata DataArray. This is produced without problem when working with Python interactively, but when working with a solution file, the tuples are all replaced with None.

This is the same part of processing the data that causes issue #54, so we probably want another way of mixing single and multiple carriers in a single DataArray.

[brynpickering]

If the metadata DataArray is converted to a pandas DataFrame, then saved as HDF, it is possible to load it back with tuples intact.

[brynpickering]

We don't have tuples in the xarray of 0.6.0, favouring comma delimited strings in DataArrays, as these can be successfully parsed in all cases.