Documentation for Pset_Superelevation.Superelevation

Issue from #269.
"Superelevation property is not specific with respect to the sign convention for the ratio measure(slope). Is a positive value slope upwards, left to right? Are positive values favorable slopes and negative values adverse slopes in terms of the righting or overturning effects of centrifugal forces?"

Current definition: "Specifies the superelevation as a ratio measure (slope) at the location of the event."

Proposed definition: "Specifies the superelevation as a ratio measure (slope) at the location of the event. The ratio is measured from the location of the event (typically on the alignment axis referenced by an _IfcLinearPlacement_) laterally to the left or right, depending on the value of the property _Side_, by dividing vertical offset (where a negative value means that the slope is downwards) with the corresponding lateral horizontal offset. E.g. a value of -0,025 to the left indicates a 2,5% (or 2,5 cm/m) downwards slope to the left.

Maybe clarification will be needed also regarding Side since left and right might not be clear in a 3D setting and one might need to use the placement axis as reference...?

Review the Pset_Superelevation.Superelevation documentation entry and compare its current definition with the proposed wording in this issue. Resolve how vertical and lateral offsets, negative slopes, and the Side property are described in a 3D setting. Done means the entry clearly defines the sign convention and left/right reference without leaving the noted ambiguity.

Written by the indexing model from the issue text.