Allow to provide strain time series in stochastic sources

In order to run long simulations without running into memory issues, one needs to chunk in time. However, we must make sure to retain time correlations, coming from

• the non-zero size of the response function kernel (10s of seconds)
• the (potentially long) correlations of the non-white strain

For the first point, we can have some "overlap" between the time chunks. For the second, the user must be able to produce the entire time series for the strain (hplus and hcross), and pass them as time series to the source class.