switched sparse format to binary trees

[Patrick Alken]

I received a bug report from someone who found that the
gsl_spmatrix_set() function did not properly detect duplicate elements
(ie it could set the same (i,j) matrix element to more than 1 value).

This was related to the linear array scheme I had originally
implemented, which made it very infefficient to find duplicates (ie: a
linear search was required each time).

So I've changed the storage scheme for the triplet representation to a
binary tree (specifically the AVL balanced binary tree). I took code
from the GNU libavl library for the tree manipulation stuff.

This actually makes a huge speed improvement in duplicate detection and
element retrieval for large sparse matrices. There may however be a
performance hit since a new tree node must be allocated each time an
element is added to the matrix. But I believe the tradeoff is worth it.

Anyone using gsl_spmatrix in their code will need to recompile since the
binary format of that struct has changed.

Patrick