Raise NotImplementedError for policies that are not implemented in the cost functions

[sakshamsoni97]

Currently implemented policies are 'No_Measure' and 'Lockdown'.

We want to gradually implement - 'No_Measure', 'Restrict_Mass_Gatherings', 'Mass_Gatherings_Authorized_But_Others_Restricted', 'Restrict_Mass_Gatherings_and_Schools', 'Authorize_Schools_but_Restrict_Mass_Gatherings_and_Others', 'Restrict_Mass_Gatherings_and_Schools_and_Others' and 'Lockdown'

Any other strings should give an error.

[bro-opt]

@MichaelLLi , I've implemented a bug fix for that, with example.py providing you with a basic unit test to see if implemented.

Commit id:
0874f4a