Specify when entity should prefix the variable name

[MaxGhenis]

Many variables in OpenFisca UK have entity in the name, e.g. household_net_income, but some like age don't. There are pros and cons to being explicit always, but let's define guidelines for when to stray if we allow straying.

[nikhilwoodruff]

OK, so I think:

• An immediate solution for consistency/development going forward is to have the variable name be entity_varname, with an allowable exception for person-level variables, which are the default.
• A more elegant solution (involving modifying Core) would be to allow multiple definitions of the same variable name under different entities. Inside variables, there doesn't need to be any additional information passed, as the variable is called from the entity. In the simulation interface, we could provide a default entity version (overall or per variable).