force there to be at least three elements in time reconstruction

This should hopefully resolve issues like

...bilby/gw/likelihood.py", line 403, in generate_time_sample_from_marginalized_likelihood
    "Time posterior reconstruction failed, at least two samples "               
bilby.core.likelihood.MarginalizedLikelihoodReconstructionError: Time posterior reconstruction failed, at least two samples are required.

This should be tested more.

bilby/gw/likelihood.py

@@ -15,7 +15,7 @@ except ImportError:
     from scipy.misc import logsumexp
 from scipy.special import i0e
 
-from ..core.likelihood import Likelihood, MarginalizedLikelihoodReconstructionError
+from ..core.likelihood import Likelihood
 from ..core.utils import BilbyJsonEncoder, decode_bilby_json
 from ..core.utils import (
     logger, UnsortedInterp2d, create_frequency_series, create_time_series,