MT write simple fault calculator output to nrml

[Giuseppe Vallarelli]

We need to write the output generated by the fault calculator to nrml. The output of a fault calculator are the occurance rates (evenlyDiscretizedIncrementalMFD). A nrml example can be found here https://github.com/gem/nrml/blob/master/schema/examples/source-model.xml . Also see https://github.com/g-weatherill/gw_misc_code/blob/master/fault_calculator/simple_fault_Myanmar_trial1.yml#L79 for the data source.

Example of simple fault: https://github.com/gem/nrml/blob/master/schema/examples/source-model.xml#L13