added fancy annotationarray indexing

[bmcfee]

Implements #112

• [x] documentation
• [x] unit tests

[bmcfee]

I think this is ready for CR.

@ejhumphrey ?

[bmcfee]

[rabitt]

This lgtm. My only comment would be to add documentation somewhere that outlines exactly what kinds of indexing can be done, both generically and with the examples you already give.

[bmcfee]

Thanks for the CR. I expanded the docs a little, should be good to go.