Make piecewise polynomial.
pp = mkpp(breaks,coefs)
puts together a pp function from the breaks and coefficients input or
requested. The number l of polynomial pieces is determined as
l := length(breaks)-1 .
The order k of the pp is obtained as
k := length(coefs)/l ,
and this ratio had better be an integer.
See also UNMKPPGQE3BU, PPVAL , SPLINE .