Unify member names in heaps (#299)

The following renamings are made.

Public members:
- UnderFlowPriorityError -> PriorityUnderflowError
("underflow" is only one word)

Private members:
- bubble_up() -> bubbleUp()
- bubble_down() -> bubbleDown()
- second_child() -> secondChild()
- makeroot() -> makeRoot()
- relocate_last() -> relocateLast()
- data -> _data
- boxes -> _boxes

#define LEMON_VERSION "@PROJECT_VERSION@"

#cmakedefine LEMON_HAVE_LONG_LONG 1

#cmakedefine LEMON_HAVE_LP 1

#cmakedefine LEMON_HAVE_MIP 1

#cmakedefine LEMON_HAVE_GLPK 1

#cmakedefine LEMON_HAVE_CPLEX 1

#cmakedefine LEMON_HAVE_CLP 1

#cmakedefine LEMON_HAVE_CBC 1