Small edit to adapt to OQ changes, fixes broken quakeml test

In the last few months OQ has made some API changes to its ground motion model library, as explained here: #25. This has caused some breakages in this library (and other libraries that use OQ's GMMs for this purpose). At present, shakyground2's shakemap calculator is using the openquake.hazardlib.contexts.ContextMaker class for preparing the inputs to the GMM, but calling the GMMs themselves according to the old style API. After some user feedback, OpenQuake's developers have ensured that OpenQuake's old style GMM remains backward compatible for the time being, so only one small change was needed to keep shakyground2 working with the 3.12.0 release of OpenQuake. This is implemented here along with some minor cleanup of formatting to address a line too long formatting error from Flake8.

Also, a small change in the precision of the magnitude and location of one of the GEOFON test events seemed to have occurred, so the test have been edited to fix this too.

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