bfs_all modules for graph and digraph modified.

Bugs have been fixed.
Testing module for dijkstra's algo added.
bfs_search module for undirected graphs added.
bfs_search function for digraphs added.
maxdepth feature for bfs_search modules added.
Bug in bfs_all for undirected graphs fixed.
bug in bfs_all for digraphs fixed.

docstrings updated appropriately.
minor change made.
dfs_search functions for graph & digraph added.

dfs_all function of graphs and digraphs have been removed.
eccentricity function for graphs added.

bug in bfs_all for graphs fixed.
minor changes made.
bug in graph_edgelist module fixed.
diameter and radius measures added for graphs.

Test cases not yet written
periphery and centre functions for graphs added.

Test cases not written yet.
Test module for distance measures of graphs added.

Execution time is significantly high for this test module.
minor change committed
bug in digraph_edgelist module fixed.
module for distance measures of digraph added.

Corresponding testing module also added which takes significant execution time due to random sampling.
implementation for eccentricity changed.
bug in wgraph_edgelist module fixed.