Filter out non-indexed/metadata fields when indexing

[seanmacavaney]

fixes #266

Also tried this fix with the example in #266 and it worked.

[cmacdonald]

Whats the overhead of a new level of generator function for all IterDict indexing?

[seanmacavaney]

There's naturally a bit of overhead in the iteration itself:

With filter: 2.53s+/-0.01
Without filter: 2.43s+/-0.02

(On antique, 5 samples)

But the difference is negligible compared to the duration and variation of the entire indexing process:

With filter: 25.03+/-0.18
Without filter: 25.00+/-0.22

[cmacdonald]

Thanks Sean