Extend Subnets->fetch_section_subnets() to accept $field and $value arguments.

Mirrors the usage of the PDO findObjects function.

    fetch_section_subnets ($sectionId, $field = false, $value = false, $result_fields = "*")

Usage Examples:

Fetch all folders from Section 1:

    $folders = fetch_section_subnets(1, 'isFolder', 1);

Fetch all subnets from Section 2 in vrfId 10:

        $subnets = fetch_section_subnets(2, 'vrfId', 10);