Add support for .empty to expressions, and some docs improvements

[rmuir]

Its possible to treat missing values as something else other than zero today, but its not intuitive: e.g. you can do it with something like doc['field'].count() == 0 ? 500 : doc['field'].

There are two problems with this:

• aggregate methods on fields don't seem to be documented anywhere.
• its not as intuitive as .empty, making it inconsistent with other scripting languages.

I think we should fix both of these: we should document count()/max()/avg()/sum()/min()/etc so that people can work better with multi-valued fields, and we should syntactic sugar (.empty) to make handling empty cases easier.

[rmuir]

Here is a quick hack. It required some refactoring, as we didnt really have things logically organized to support "variables" other than .value. @jdconrad can you look at this?

[rmuir]

as far as the re-organization, i had in mind we may want to support some other "variables" that the other script engines support besides just .value and .empty. For example, maybe simple additions like .lat/.lon so that geo fields can start to work with expressions too?

[rmuir]

Another TODO: in the docs. They emphasize Only numeric fields may be accessed but i don't think its necessarily clear that date fields will work too. I doubt most people immediately jump to the conclusion that date fields are numbers too, except us.

[jdconrad]

LGTM. Thanks for adding this!