Fix bugs in load_budget

Fixes two bugs in load_budget

1. Inheritance now works when inheriting a custom budget that is not one of the canonical budgets if the yaml file inheriting it is not in the same directory as the custom budget.
2. According to the help and readme, a yaml file which does not specify a budget to inherit should be loaded into the aLIGO budget. A bug in the logic was causing this to raise an error but is now working again. However, now that inheritance is working I propose that it no longer be possible to load a yaml file which doesn't specify which budget to use.