Wavemodel add editing functions (add_node, remove_node, add_edge, remove_edge)

[DerFred3]

Also adds a is_modified-bit to the WaveModel-structure, which is set after one of the editing functions has been called. The is_modified-bit determines, whether the wavelet-matrix, sequence and bitmap need to be rebuild when actively querying from these structures.

Closes #45 and #50.