intermediate commit: verify checksums
added checksum calculation according to RFC 1071 for testing purposes, added unit test framework for checksum testing
revised tcpv4 and udpv4 checksum calculation and added reference implementation for checksums from IETF RFC 1071
c_types_pktclassifier.c => studying invalid length value returned from get_pkt_len()
Fixing more clas_state->len bugs during push/pop operations
ryu tests with pop_mpls and checksum calculation now succeed