[#365,!194] kea-dhcp4 can be configured to calculate T1 and T2

doc/guide/dhcp4-srv.xml
    Updated user guide with new parameters and details on rules

src/lib/cc/data.cc
    DoubleElement::toJSON(std::ostream& ss) - modified to ensure whole number
     values are suffixed with ".0"

src/lib/utils/doubles.h - new file
   provides a function for equating doubles within a given tolerance

src/lib/cc/simple_parser.*
    SimpleParser::getDouble() - new method for fetching real number
    parameters as DoulbeElements

src/lib/dhcpsrv/network.*
    Added members and support for new paramters to Network4:
        calculate_tee_times_, t1_percent_, t2_percent_

src/lib/dhcpsrv/parsers/dhcp_parsers.cc
src/lib/dhcpsrv/parsers/simple_parser4.cc
    Added support for new parameters to Subnet4ConfigParser

src/bin/dhcp4/dhcp4_lexer.ll
src/bin/dhcp4/dhcp4_parser.yy
src/bin/dhcp4/json_config_parser.cc
    Added parser support for calculate-tee-times, t1-percent, and t2-percent

src/bin/dhcp4/dhcp4_srv.*
    Dhcpv4Srv::setTeeTimes(lease, subnet, resp) - new method for
    determining T1 adn T2 values

src/bin/dhcp4/json_config_parser.cc

@@ -1,4 +1,4 @@
-// Copyright (C) 2012-2018 Internet Systems Consortium, Inc. ("ISC")
+// Copyright (C) 2012-2019 Internet Systems Consortium, Inc. ("ISC")
 //
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
@@ -546,7 +546,10 @@ configureDhcp4Server(Dhcpv4Srv& server, isc::data::ConstElementPtr config_set,
                  (config_pair.first == "server-hostname") ||
                  (config_pair.first == "boot-file-name") ||
                  (config_pair.first == "server-tag") ||
-                 (config_pair.first == "reservation-mode")) {
+                 (config_pair.first == "reservation-mode") ||
+                 (config_pair.first == "calculate-tee-times") || 
+                 (config_pair.first == "t1-percent") || 
+                 (config_pair.first == "t2-percent")) {
                 continue;
             }