MAINT: multiple vega-lite versions within Altair

[jakevdp]

This PR rearranges some things to allow multiple versions of Vega-Lite to be supported in a single Altair release (addresses #375). So, for example, you could do as normal

import altar as alt

alt.Chart(data).mark_point().encode( ... )

or equivalently,

import altair.v1 as alt

alt.Chart(data).mark_point().encode( ... )

Currently v1 is the only option available, but this reorganization of the code will make it easy to add additional versions as well.

[jakevdp]

Changes look big, but the bulk of it is just rearranging existing code.

[jakevdp]

Tests pass! @ellisonbg, would you mind taking a quick look? One thing I'm not certain on is the mirroring of altair.v1 submodules in the altair namespace. Does the way I did this make sense to you?

[ellisonbg]

Working on reviewing this:

We have an example notebook that looks at api.AggregateOp.values, but AggregateOp is no longer in api. Do we want this to be in the public API. I am fine removing it from the examples or exporting it in api.

I am seeing a test failure locally:

======================================================== FAILURES =========================================================
_________________________________________________ test_hastraits_required _________________________________________________

    def test_hastraits_required():
    
        class Foo(jst.JSONHasTraits):
            _required_traits = ['name']
            name = jst.JSONString()
            age = jst.JSONNumber()
    
        f1 = Foo(name="Sue", age=32)
        f2 = Foo(age=32)
    
        # contains all required pieces
        f1.to_dict()
    
        with pytest.raises(T.TraitError) as err:
            f2.to_dict()
>       assert err.match("Required trait 'name' is undefined")
E       AttributeError: 'ExceptionInfo' object has no attribute 'match'

altair/v1/schema/_interface/tests/test_jstraitlets.py:132: AttributeError
===================================== 1 failed, 381 passed, 4 skipped in 8.57 seconds =====================================

@jakevdp you were asking about mirroring the sub are you asking about how you are importing expr and tutorial in v1.__init__. If we want to recommend people to do the following imports, then I think we need to:

import altair as alt
import altair.v1 as alt

So +1 on that unless I am misunderstanding something.

[ellisonbg]

Going to merge and iterate...we can make adjustments later if needed.