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

 prefix=@prefix@

 exec_prefix=@exec_prefix@

 libdir=@libdir@

 includedir=@includedir@

 Name: @PACKAGE_NAME@

 Description: Library for Efficient Modeling and Optimization in Networks

 Version: @PACKAGE_VERSION@

 Libs: -L${libdir} -lemon @GLPK_LIBS@ @CPLEX_LIBS@ @SOPLEX_LIBS@ @CLP_LIBS@ @CBC_LIBS@

 Cflags: -I${includedir}