SYSML21 — Degree to Radian conversion imprecise

The definition of 'pi' provided in TrigFunctions::pi (KerML) also includes an invariant that specifies PI to 20 significant digits.

However, in SI.sysml where we define 'degree', the conversion factor is listed as:

attribute <'°'> degree : AngularMeasureUnit { :>> unitConversion: ConversionByConvention { :>> referenceUnit = rad; :>> conversionFactor = 1.745329E-02; :>> isExact = false; } } // conversionFactor should become pi/180

It appears that this was intended to be a "TODO" that was never addressed!

Having the conversion factor from degrees to radians only listed to "single precision" will be a severe problem for many engineering applications. We should instead depend on the actual "pi/180" expression as stated in the trailing comment