BandSPDLinSOE is class which is used to store a banded symmetric system
with ku superdiagonals. The A matrix is stored in a
1d double array with (ku+1)*n elements, where n is the size of the
system. A_{i,j} is stored at location (ku+1+i-j) +
j*(ku+1), where i and j range from 0 ton-1, i.e. C
notation. For example when n=5, ku = 2:
\left[
\begin{array}{ccccc}
a_{0,0} & a_{0,1} & a_{0,1} & 0 & 0