Add raw CRC

[PhyxionNL]

As raw() doesn't return the CRC.

[matthijskooijman]

Thanks for your PR, apologies for not responding to it earlier...

I've looked at the patch and it seems useful, but I wonder if it would maybe be better/simpler to just include the checksum in the buffer, and exclude it from parsing, rather than keeping a second buffer just for the CRC. Also, there are some changes requested for DSMR 3.0 support without a checksum (see #18), which would probably be useful to handle at the same time.