Allow duplicate dates in TimeSpanMap
There are cases when the same date is used twice in TimeSpanMap.
One such example is in TimeSpanEstimatedTroposphericModel, where there
are two methods addTroposphericModelValidBefore and addTroposphericModelValidAfter
which are both called from AbstractOrbitDetermination (in the unit tests and
in the tutorials) with the exact same epoch.
This was explicitly forbidden in TimeSpanMap API, but it seems to be useful, so
it should be supported.
Currently, depending on how the changes for issue #833 (closed) are implemented, this makes some test fail as tropospheric models are not set up properly.