Assigning default function type from spec

[pjsier]

Fixes some lingering issues from #161. I noticed that when I load a style where the type wasn't explicitly identified on a data driven style that it would default to categorical. Instead of this, if an uploaded style doesn't have a type property this checks the fieldSpec to determine what it should be.

According to the style spec details on functions, any property that supports interpolated must be numeric and defaults to exponential, and any property that is numeric and supports functions (checked elsewhere) must support interval. I'm using that logic in getFieldFunctionType to determine which to assign, and it's worked on styles I've tested for exponential, interval, and categorical