$(D_KEYWORD true) if this is a multicast address, $(D_KEYWORD false) otherwise.
assert(Address(address4("224.0.0.3").get).isMulticast()); assert(Address(address6("ff00::").get).isMulticast());
$(D_PSYMBOL Address4.isMulticast), $(D_PSYMBOL Address6.isMulticast).
Determines whether this address' destination is a group of endpoints.