Simplify flow of valuesAt()
Simplify flow of valuesAt()

This change reverts a recent fix and fixes the issue in a different way.

The issue resulted from inaccuracy of floating point operations.
If we go through the T-Digest structure left-to-right, summing deltas
until we reach the last centroid, we should get the same offset as
starting from the end and going back half the last weight.
However, the results differ slightly. That caused an error.

Previously, the issue was addressed by excluding the particular problematic
edge case from its usual flow. However, the solution was incomplete:
- it only addressed the case when the former result was greater than the latter
- it only addressed the issue of inaccuracy around the last centroid

After this change, a comparison is removed. As a result, in this rare case,
there is a possibility that the interpolation is performed based on
different edge values than the algorithm says (i.e. the interpolation
uses the last mean and max value instead of two last means).
However, it should only slightly affect the result, which in both cases
lies very close to the last mean.

Actually, the former fix led to symmetric situation (i.e. potential
interpolation between the last two centroids when it should be between
the last centriod and max).

Moreover, the choice of the right area for interpolation is prone to
floating point inaccuracy throughout the T-Digest structure.
It does not apply only to the last centroid but potentially appears
whenever the searched offset falls very close to centroid middle.
This issue wasn't addressed neither by the former fix nor by this fix.
My assumption is that it is generally acceptable based on the fact that
the resulting value is very close to the centroid's mean in any case.

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