Bug when two particles have D=0

[Jintram]

Then the method that decides on the minimum pair size fails:

jintram@jintram-Aspire-7745G:~/jul2012_gfrd_git/samples/cluster$ python run.py 7 4 data.out
v23jun2011.3
Seed: 850
Constants: sigma=1e-08; D=1e-12; world_size=4.472135955e-07; k1=1.25663706144e-19; k2=100.0; spacing=1e-13; N=7; runs=4; kd/k1=1.0; f=0.0035
Traceback (most recent call last):
  File "run.py", line 292, in <module>
    outFile.write(str(single_run(N, LOGGING)) + '\n')
  File "run.py", line 266, in single_run
    s.step() # make eGFRD step
  File "/home/jintram/jul2012_gfrd_git/egfrd.py", line 445, in step
    f(self, domain, [domain.domain_id])         # fire the correct method for the class (e.g. process_single_event(self, Single))
  File "/home/jintram/jul2012_gfrd_git/egfrd.py", line 1760, in process_single_event
    domains = [self.make_new_domain(single)]
  File "/home/jintram/jul2012_gfrd_git/egfrd.py", line 1588, in make_new_domain
    domain = self.try_pair (single, obj)
  File "/home/jintram/jul2012_gfrd_git/egfrd.py", line 2296, in try_pair
    testShell = try_default_testpair(single1, single2, self.geometrycontainer, self.domains)
  File "/home/jintram/jul2012_gfrd_git/egfrd.py", line 138, in try_default_testpair
    return SphericalPairtestShell           (single1, single2, geometrycontainer, domains)
  File "/home/jintram/jul2012_gfrd_git/shells.py", line 1303, in __init__
    [])
  File "/home/jintram/jul2012_gfrd_git/shells.py", line 1220, in determine_possible_shell
    min_radius = self.apply_safety(self.get_min_radius())
  File "/home/jintram/jul2012_gfrd_git/shells.py", line 1309, in get_min_radius
    return self.get_min_pair_size()
  File "/home/jintram/jul2012_gfrd_git/shells.py", line 296, in get_min_pair_size
    dist_from_com1 = self.r0 * D_1 / self.D_tot      # particle distance from CoM
ZeroDivisionError: float division by zero