Change default behaviour of the Unblinder when calculating limits

[JannisNe]

Describe the bug
The injection time is needed to multiply it with the mean luminosity to get the mean energy emitted in neutrinos. The Unblinder calculates this injection time using TimePDF.raw_injection_time() which gives the overlap of the source with the dataset.

Expected behavior
This is an at least controversial way of doing it since for example for a box time pdf we would rather want the box length and not the overlap of the sources with the dataset.

Solution
We could define a property of the TimePDF`` called maybe length``` that return the injection length. This could then be used by the Unblinder.

[mlincett]

I think this issue is now obsolete as the calculation of the energy in the source frame is no longer supported after #236.

Shall we close it?