for simple cases, "concat curves" from several NURBS curves will result
in a NURBS curve.

Simple case is one so:
* either degrees of all curves is the same
* or, if some curve has smaller degree than others, it has to have
  number of control points = it's degree + 1
* end points of curves must coincide
* end control points of curves must coincide
* weights of the end control points must be equal
* multiplicity of boundary knots (at merging points) must be all
  degree+1.