Implementation of Stafford (2014) source-scaling relation

[pstafford]

This implementation includes magnitude-to-area relations only and simply replicates the structure of other source-scaling relations already in oq-hazardlib

[micheles]

Thanks for your contribution. We are happy to accept new MSR classes, but our policy is to have tests for each new feature. You can have a look at this document that gives some instructions to contributors: https://github.com/gem/oq-hazardlib/blob/master/doc/implementing-new-gsim.md. Since you are contributing a new MSR and not a GSIM, for the tests you can look at the existing ones for other MSRs: https://github.com/gem/oq-hazardlib/tree/master/openquake/hazardlib/tests/scalerel

[g-weatherill]

I am in the middle of a substantial refactor of the whole MSR library, of which this will be one new addition. A new PR will be opened in due course with everything, including tests added. I recommend to keep this PR open for comment until I have made the refactor and then close it once the code is moved into the new PR.

[mmpagani]

Thanks Graeme for your message. I kindly ask you to coordinate with us for any major refactoring as you are anticipating in your last message. Regarding Peter's PR I would suggests he proceeds with the preparation of tests.