add support for simplified legends

[brharrington]

The default legends just summarize the expression and are
often hard to read and get truncated. This change adds a
helper that tries to generate a simplified legend if the
user has not set the legend explicitly. It will look at
the full set of expressions being rendered and try to
emphasize differences where possible.

Example:

  Expr:
       nf.cluster,www-main,:eq,
       name,apache.http.request,:eq,:and,
       :dist-stddev

Before:
       sqrt(
         (
           (
             (
               (statistic=count) and (
                 (nf.cluster=www-main) and (name=apache.http.request)
               )
               *
               (statistic=totalOfSquares) and (
                 (nf.cluster=www-main) and (name=apache.http.request)
               )
             )
             -
             (
               (statistic in (totalAmount,totalTime)) and (
                 (nf.cluster=www-main) and (name=apache.http.request)
               )
               *
               (statistic in (totalAmount,totalTime)) and (
                 (nf.cluster=www-main) and (name=apache.http.request)
               )
             )
           )
           /
           (
             (statistic=count) and (
               (nf.cluster=www-main) and (name=apache.http.request)
             )
             *
             (statistic=count) and (
               (nf.cluster=www-main) and (name=apache.http.request)
             )
           )
         )
       )

 After:
       apache.http.request

This can be disabled by setting:

atlas.eval.graph.simple-legends-enabled = false