Fix bugs in load_budget
Fixes two bugs in load_budget
- 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.
- 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.