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

[#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