Issue 4468/1: stencil-integrate.cc: root out "slopes"

Issue 4468/1: stencil-integrate.cc: root out "slopes"

They are quite unsuitable for the computational geometry performed here.
Their use rendered the code inefficient, obtuse, and unreliable.

Contains several other fixes like removing duplicate calculations and
wrong values.  The C++ <complex> module is no longer used.  It had only
been used for calculating sin and cos in an obscure manner.