Bug fix: The error that showed itself when the if-sentences in
BSplineBasis1D::buildBasisMatrix were used was caused by a wrong
assumption of the code using the produced matrix. The assumption was
that the sparse matrix had a fixed number of "non-zero" elements: using
insert also on zero elements ensured a correct number of "non-zeros".
Not inserting the zero elements made the assumed number of "non-zeros"
incorrect.