Capture zero division error when all column data is nan

[fsaad]

RuntimeError: Subprocess failed: Traceback (most recent call last):
  File "/scratch/fsaad/cgpm/cgpm/utils/parallel_map.py", line 50, in process_input
    ok, fx = True, f(x)
  File "/scratch/fsaad/cgpm/cgpm/crosscat/engine.py", line 37, in _intialize
    state = State(X, rng=gu.gen_rng(seed), **kwargs)
  File "/scratch/fsaad/cgpm/cgpm/crosscat/state.py", line 92, in __init__
    self.crp.transition_hyper_grids([1]*self.n_cols())
  File "/scratch/fsaad/cgpm/cgpm/mixtures/dim.py", line 175, in transition_hyper_grids
    [x for x in X if not math.isnan(x)], n_grid=n_grid)
  File "/scratch/fsaad/cgpm/cgpm/primitives/crp.py", line 150, in construct_hyper_grids
    grids['alpha'] = gu.log_linspace(1./len(X), len(X), n_grid)
ZeroDivisionError: float division by zero