-getavg
template: pers?clnt=<name|pass>&com=getavg&ftr=<ftr_pattern>[&usr=<usr>]
          Order of query params is not important. Query param 'usr'
          is optional. If ommited, all users are considered.
pattern : * | name[.*], where name is a path expression.
descript: for the specified feature(s) and (maybe) user, the average
          value of the numeric feature values 'num_value' is returned.
          If no feature in DB matches the pattern, or if user does not
          exist, the result will not have any 'row' elements (200 OK
          will still be returned).
example : pers?clnt=name|pass&com=getavg&ftr=laptop.weight&usr=kostas
          pers?clnt=name|pass&com=getavg&ftr=speed.*
returns : 200 OK, 401 (fail, request error), 501 (fail, server error)
200 OK  : in this case the response body is as follows
          <?xml version="1.0"?>
          <?xml-stylesheet type="text/xsl" href="/resp_xsl/average_featureval.xsl"?>
          <result>
              <row><avg>average value</avg></row>
              ...
          </result>
comments: the reference to the xsl file allows to view results
          in a web browser. In case the response body is handled
          directly by an application and not by a browser, this
          reference to xsl can be ignored.