layout: Run entity are parsed as strings

[adelavega]

It looks like the entity run is being extracted as a string, not an integer. The zero padding is correctly dealt with, but it is stored as a string.

I could have sworn we deal with this before so that it would read run in as a integer, but it looks like even in the tests, it expects strings.

[adelavega]

Looks like in bids.json, run should have dtype set to int.

Changing this breaks other grabbit stuff (other functions seems to be mostly tested on strings)

I am incorrect in recalling at some point it was reading in runs as ints though?

[tyarkoni]

Fixed in #236.