Implement an RDF-LDF can_answer($pattern) method

[phochste]

 my $ldf = RDF::LDF->new(url => $url);
 if ($ldf->can_answer($pattern)) {
       ... do stuff ...
 }

In the LDF protocol not all variables could be available for querying. The hydra:variable section in the LDF controls defined the mapping of requests parameters to LDF queries.

[phochste]

LDF context should contain hydra:required for all parameters that must be provided in the pattern query