Feat/pypangraph junctions

[mmolari]

The aim of this PR is to add functionalities to PyPangraph to analyze core-genome junctions.

Core-genome junctions are defined as regions in the graph separated by two adjacent flanking core blocks.

The main additions of the PR adds:

• a class BackboneJunctions that serves as the interface to functions for junction analysis
• the method stats of this class, returning summary statistics for each junction.
• the method positions that computed junction coordinates in genomes frames of reference.
• the method sequences, that extracts from the graph unaligned sequences for each junction.
• updates to the documentation