Add optional dependency class

[ColCarroll]

This is a way to use methods from pymc3 or altair when it is clear the user wishes to use such a method. For example, if the user passes a pymc3 trace, I think it would be fair to use a method from arviz.compat.pymc3.

I also plan to use this to implement some methods using arviz.

[coveralls]

Pull Request Test Coverage Report for Build 165

• 0 of 16 (0.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.8%) to 77.763%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
arviz/compat/init.py	0	3	0.0%
arviz/compat/optional_dep.py	0	13	0.0%

Totals
Change from base Build 163:	-0.8%
Covered Lines:	1196
Relevant Lines:	1538

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💛 - Coveralls

[ColCarroll]

also just turned off coveralls comments. can re enable if you find them useful. there are no tests here because it is quite difficult to test, but I can add those if you'd like (I think pymc3 enforces that matplotlib is not required).

[aloctavodia]

I like the coveralls comments turned off and I am OK about skipping the tests. I don't really understand what this new class is for, but I guess I will when you use it for something concrete, so feel free to merge this.

[ColCarroll]

Sorry for not explaining better!

My immediate use case is to implement something like traceplot_altair, as well as to get rid of some pymc3 specific code: it seems like arviz.trace_to_dataframe(trace) should call pymc3.trace_to_dataframe if trace is a multitrace from pymc3.

[aloctavodia]

OK I see now, thank you. I do not remember exactly what I changed, but I think ArviZ's trace_to_dataframe is not the same that the one from PyMC3. I will check it tomorrow, dinner is waiting! :-)