documentation issue at IfcSimplePropertyTemplate

There is a contradiction between the definition at IfcSimplePropertyTemplate for how units and measure types are handled for properties with bounded values, compared with IfcPropertyBoundedValue.

In IfcPropertyBoundedValue the where rules determine, that the measure type of all three potential values have to be the same (the documentation itself is not that explicit), but at IfcSimplePropertyTemplate the documentation requests for a second measure type for the upper bound.

Proposed solution:

1) Improve documentation at IfcPropertyBoundedValue

> An _IfcPropertyBoundedValue_ defines a property which has a maximum of three (numeric or descriptive) values assigned, the first value specifying the upper bound, the second value specifying the lower bound and the third value specifying a set point value. All values shall have the same measure type. The _IfcPropertyBoundedValue_ defines an open or closed value bound (min-max combination with a potential set point) depending on the provision of the optional UpperBoundValue and LowerBoundValue:
>
> - if the LowerBoundValue and the UpperBoundValue are both provided, then it defines a closed bound.
> - If either the LowerBoundValue or the UpperBoundValue is not provided, then it indicates an open bound (either a minimum value or a maximum value). The interval is by definition inclusive, the values provided for the LowerBoundValue or the UpperBoundValue are included in the interval.
>
> The unit is handled by the Unit attribute ...

2) Improve documentation at IfcSimplePropertyTemplate

in attribute definition of PrimaryMeasureType
- replace description with "P_BOUNDEDVALUE: determining the measure type of IfcPropertyBoundedValue.LowerBoundValue, UpperBoundValue and SetPointValue"

in attribute definition of SecondaryMeasureType
- remove the definition "P_BOUNDEDVALUE: determining the measure type of IfcPropertyBoundedValue.UpperBoundValue"