Improve memory consumption in the GMF calculation

The event based calculator stores for each rupture all the ground motion values, even the zeros one: this is the reason why it is using more memory than needed. Moreover the zeros are discarded at saving time, so they are stored without any good reason. This pull request discards the zeros before, by improving significantly the memory consumption (a factor of 9 in the case of the Japan model). It also adds a missing test for the compute_gmf routine.