gwpy-plot crashes on long q-transform requests

Created by: areeda

The issue is caused by a fixed small time resolution value resulting in unreasonable intermediate image size. Error is:

    **self.qxfrm_args)
  File "/Users/areeda/py_envs/gwpy/lib/python2.7/site-packages/gwpy/timeseries/timeseries.py", line 1793, in q_transform
    new = Spectrogram(interp(out.times.value, freqs2 + fres/2.).T,
  File "/Users/areeda/py_envs/gwpy/lib/python2.7/site-packages/scipy/interpolate/interpolate.py", line 307, in __call__
    z = fitpack.bisplev(x, y, self.tck, dx, dy)
  File "/Users/areeda/py_envs/gwpy/lib/python2.7/site-packages/scipy/interpolate/_fitpack_impl.py", line 1045, in bisplev
    z, ier = _fitpack._bispev(tx, ty, c, kx, ky, x, y, dx, dy)
RuntimeError: Cannot produce output of size 300000x10327 (size too large)```