Discover doesn't simultaneously handle var and heterogeneous types

[mrocklin]

Failing case

In [6]: discover([[1, 2, 3.0], [4.0, 5], [None, 6.0, 7]])
Out[6]: dshape("((int64, int64, float64), (float64, int64), (null, float64, int64))")

Should be

3 * var * ?float64

[cpcloud]

this is also a valid type signature:

In [8]: ds.DataShape(3, ds.var, ds.Option(ds.numeric))
Out[8]: dshape("3 * var * ?{numeric}")

[cpcloud]

If someone comes along needing this then we'll implement it.