Log Message:
Changes 4.51:
Singleton and dense phases were implemented on computing
LU-factorization with Gaussian elimination. The singleton phase
is a feature that allows processing row and column singletons
on initial elimination steps more efficiently. The dense phase
is a feature used on final elimination steps when the active
submatrix becomes relatively dense. It significantly reduces
the time needed, especially if the active submatrix fits in CPU
cache, and improves numerical accuracy due to full pivoting.

The API routine glp_adv_basis that constructs advanced initial
LP basis was replaced by an improved version, which (unlike the
old version) takes into account numerical values of constraint
coefficients.

The proximity search heuristic for MIP was included in the GLPK
integer optimizer glp_intopt. On API level the heuristic can be
enabled by setting the parameter ps_heur in glp_iocp to GLP_ON.
This feature is also available in the solver glpsol through
command-line option '--proxy'.

A bug was fixed that caused numerical instability in the FPUMP
heuristic.