gapFind/Fill, glpk (lazy) interfaces, test files

The gapFind/gapFill methods were added. Also the glpk interfaces with
and without lazy loading (note that the latter is faster, in some cases
even faster than gurobi).
Unit tests for the solvers, gapFill, and gapFind were added, as well as
the necessary files in the models folder.
Merge pull request #16 from cdanielmachado/marta-branch

merging marta-branch into master