+ representing the criteria for filtering.
+ example : filter = { 'hostname' : '*.edu' , site_id : [34,54] }
+ Whether the filter represents an intersection (AND) or a union (OR)
+ of these criteria is determined by the join_with argument
+ provided to the sql method below
+
+ Special features:
+
+ * a field starting with the ~ character means negation.
+ example : filter = { '~peer_id' : None }
+
+ * a field starting with < [ ] or > means lower than or greater than
+ < > uses strict comparison
+ [ ] is for using <= or >= instead
+ example : filter = { ']event_id' : 2305 }
+ example : filter = { '>time' : 1178531418 }
+ in this example the integer value denotes a unix timestamp
+
+ * if a value is a sequence type, then it should represent
+ a list of possible values for that field
+ example : filter = { 'node_id' : [12,34,56] }
+
+ * a (string) value containing either a * or a % character is
+ treated as a (sql) pattern; * are replaced with % that is the
+ SQL wildcard character.
+ example : filter = { 'hostname' : '*.jp' }
+
+ * fields starting with - are special and relate to row selection, i.e. sorting and clipping
+ * '-SORT' : a field name, or an ordered list of field names that are used for sorting
+ * these fields may start with + (default) or - for denoting increasing or decreasing order
+ example : filter = { '-SORT' : [ '+node_id', '-hostname' ] }
+ * '-OFFSET' : the number of first rows to be ommitted
+ * '-LIMIT' : the amount of rows to be returned
+ example : filter = { '-OFFSET' : 100, 'LIMIT':25}
+
+ A realistic example would read
+ GetNodes ( { 'hostname' : '*.edu' , '-SORT' : 'hostname' , '-OFFSET' : 30 , '-LIMIT' : 25 } )
+ and that would return nodes matching '*.edu' in alphabetical order from 31th to 55th