Support of grouping for GtLtPlugin

[espdev]

Hello,

Currently, we cannot use () groups with GtLtPlugin. The following examples raise NotImplementedError: GtLtNode exception:

(year:>2001 AND year:<=2003)
year:(>2001 <=2003)

It would be useful in the following query with two ranges, for example:

(year:>2001 year:<=2003) (year:>2005 year:<=2010)