Trac #29660: Move radius, diameter and eccentricity methods from gene…

…ric_graph.py to graph.py and digraph.py

Currently radius, diameter and eccentricity computation methods are in
generic_graph.py file. Since graph and digraph both have different
algorithms for their computation. So it would be nice to have these
method separately for graph and digraph in respective files.

URL: https://trac.sagemath.org/29660
Reported by: gh-vipul79321
Ticket author(s): Vipul Gupta
Reviewer(s): David Coudert