Missing values imputation implementation

[jsalvatier]

This is actually ready to merge.

[twiecki]

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ERROR: pymc3.tests.test_shared.test_deterministic
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Traceback (most recent call last):
  File "/home/travis/miniconda/envs/testenv/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/travis/build/pymc-devs/pymc3/pymc3/tests/test_shared.py", line 14, in test_deterministic
    y = pm.Normal('y', 0, 1, observed=X)
  File "/home/travis/build/pymc-devs/pymc3/pymc3/distributions/distribution.py", line 19, in __new__
    return model.Var(name, dist, data)
  File "/home/travis/build/pymc-devs/pymc3/pymc3/model.py", line 158, in Var
    var = ObservedRV(name=name, data=data, distribution=dist, model=self)
  File "/home/travis/build/pymc-devs/pymc3/pymc3/model.py", line 410, in __init__
    data = as_tensor(data, name,model,distribution.dtype)
  File "/home/travis/build/pymc-devs/pymc3/pymc3/model.py", line 369, in as_tensor
    data = pandas_to_array(data).astype(dtype)
ValueError: setting an array element with a sequence.
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ERROR: Test that samplers correctly create non-blocked compound steps.
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Traceback (most recent call last):
  File "/home/travis/miniconda/envs/testenv/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/travis/build/pymc-devs/pymc3/pymc3/tests/test_step.py", line 58, in test_non_blocked
    nuts = pm.NUTS(blocked=False)
  File "/home/travis/build/pymc-devs/pymc3/pymc3/step_methods/arraystep.py", line 43, in __new__
    step.__init__([var], *args, **kwargs)
  File "/home/travis/build/pymc-devs/pymc3/pymc3/step_methods/nuts.py", line 61, in __init__
    scaling = guess_scaling(Point(scaling, model=model), model=model, vars = vars)
  File "/home/travis/build/pymc-devs/pymc3/pymc3/tuning/scaling.py", line 82, in guess_scaling
    return adjust_scaling(h)
  File "/home/travis/build/pymc-devs/pymc3/pymc3/tuning/scaling.py", line 86, in adjust_scaling
    return adjust_precision(s)
  File "/home/travis/build/pymc-devs/pymc3/pymc3/tuning/scaling.py", line 101, in adjust_precision
    mag = tau * 1/np.tanh(tau/bound)
FloatingPointError: invalid value encountered in divide
======================================================================
ERROR: pymc3.tests.test_tuning.test_adjust_precision
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Traceback (most recent call last):
  File "/home/travis/miniconda/envs/testenv/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/travis/build/pymc-devs/pymc3/pymc3/tests/test_tuning.py", line 11, in test_adjust_precision
    a1 = scaling.adjust_precision(a)
  File "/home/travis/build/pymc-devs/pymc3/pymc3/tuning/scaling.py", line 101, in adjust_precision
    mag = tau * 1/np.tanh(tau/bound)
FloatingPointError: invalid value encountered in divide
----------------------------------------------------------------------
Ran 406 tests in 1861.233s

[jsalvatier]

Close with #743