FindGLPK improvements (#256)

Merge

Fix and uniform the usage of Graph and Parent typedefs (#268)

- Rename Graph typedefs to GraphType in the implementation of graph
maps and MapExtender to prevent conflicts (especially using VS).
They are not public.
- Make Parent typedefs private in all classes.
- Replace Digraph with Graph in some places
(fix faulty renamings of the script).
- Use Graph and Digraph typedefs (more) consequently.

VS compatibility fix (#268)

Exploit the changes of #190 in MCF test file (#234, #190)

Support LEQ and GEQ supply constraints in dimacs-solver (#234, #219)

Bug fix in NetworkSimplex (#234)

Fix usage of sqrt() (#268)

Merge and fix

Slightly modify the interface of Circulation and Preflow (#266)
in order to synchronize them to the interface of NetworkSimplex.

Circulation:
- The "delta" notation is replaced by "supply".
- lowerCapMap(), upperCapMap() are renamed to lowerMap() and upperMap().
- Value is renamed to Flow.

Preflow:
- Value is renamed to Flow.

Support >= and <= constraints in NetworkSimplex (#219, #234)

By default the same inequality constraints are supported as by
Circulation (the GEQ form), but the LEQ form can also be selected
using the problemType() function.

The documentation of the min. cost flow module is reworked and
extended with important notes and explanations about the different
variants of the problem and about the dual solution and optimality
conditions.

Support real types + numerical stability fix in NS (#254)

- Real types are supported by appropriate inicialization.
- A feature of the XTI spanning tree structure is removed to ensure
numerical stability (could cause problems using integer types).
The node potentials are updated always on the lower subtree,
in order to prevent overflow problems.
The former method isn't notably faster during to our tests.

Separate types for flow and cost values in NetworkSimplex (#234)

Support multiple run() calls in NetworkSimplex (#234)