fixed directory structure
dijkstra class scaffold
changed docstring
changed doctring and added argument for start node
dijkstras complete
added top-level importer for modules
made bellman-ford class
created superclass for graph solvers to hold shared functionality
first draft of bellman-ford
minimum (negative) edge weight can be specified in generator
removed syntax error
allowed bellman_ford to take no arguments
made BF's solve() graph argument optional
raised actual exceptions and BF tweaks
BF works