FEAT: Add Shorrocks mobility index

[natashawatkins]

Adds the Shorrocks mobility index as described in this paper on page 13: https://www.econ.nyu.edu/user/bisina/RevisionAugust.pdf

[mmcky]

• update docs to include

[natashawatkins]

I was trying to follow the instructions here: https://github.com/QuantEcon/QuantEcon.py/tree/master/docs but wasn't able to compile the docs - was getting an import error

Is my commit sufficient though?

[natashawatkins]

Also I think we need to add documentation for the nelder_mead algorithm added by @QBatista ?

[mmcky]

probably need to add it to the index?
we do - but please don't add that in this PR. We will make a separate PR to add nelder_mead

[mmcky]

thanks @natashawatkins I will dig into this docs issue on Monday.

[jstac]

Nice work @natashawatkins. Nice code and tests.

Let's try to remember to get this in a notebook somehow, so we make it known.

@cdagnino FYI, since I seem to remember you were looking at mobility indices.

[coveralls]

Coverage decreased (-0.006%) to 94.575% when pulling 0a9c1f9 on add-shorrock into 3772fdd on master.

[mmcky]

thanks @natashawatkins I will merge this PR and submit a fix for docs