refactoring of spline

[cm314]

• revise which methods can be gathered in base class Spline from BSpline and NURBS and which function (parts) have to be virtual and implemented differently, cf. Issue147 degree elevation #172 (comment) and Issue147 degree elevation #172 (comment)
• use meaningful names, cf. Issue147 degree elevation #172 (comment) and Issue147 degree elevation #172 (comment)