Removes unused imports

pydtn/network.py

-from argparse import ArgumentParser
-from collections import defaultdict, OrderedDict
+from collections import OrderedDict
 from itertools import combinations
-import math
 import random
-import sys
 
-import simpy
-import yaml
 import networkx as nx
 
 from .core import TickProcess
 
-from .communities import types as communities
 from .routers import types as routers
 from .traces import types as traces