add ability to cache detector response to marginalize time model and us…

[ahnitz]

This implements two features into the marginalized time model.

1. it can now take a 'sample_rate' argument. If given, a different sample rate can be used for the marginalization than the intrinsic one of the data. Note it really should be higher than the data sample rate to make sense.

2. The detector response factors are now retrieved from the precalculated ones which improves the marginalization speed by ~ 50 %.