Feature: store entity metadata

[larsyencken]

Currently we store variable level metadata in the Table object, but not entity level metadata. We should decide whether we want to store it globally in a single file for the whole dataset, or whether we want to do it per table.

[larsyencken]

@danyx23 Flagging this as a problem for us to solve next week.

[larsyencken]

Thinking about this more, I think we should just store entity metadata in VariableMeta, and just adapt it so that it has the fields we want/need for categorical data too.

One problem with this approach is that whilst we can say:

table.gdp.title = 'GDP in 2011 international $'

We don't have an obvious way of doing that for fields in the index, i.e. we can't write:

table.index.agegroup.description = 'Something...'.

So there's a tiny usability problem in the Python API to solve.