Support for alternate checksum type indicated in Metadata PDU

[jphickey]

Per the CCSDS blue book 727.0-B-5, section 5.2.5 - the metadata PDU has a 4-bit "checksum type" field. The description says:

Checksum algorithm identifier as registered in the SANA Checksum Types Registry.
Value zero indicates use of the legacy modular checksum.

It looks like CF only supports the legacy checksum, because it does not check this field at all, and there appears to be only one algorithm implemented. However, this is not as significant as the large file size issue (#95) because it does not otherwise change the format, here the result will be a simple failure of the validation check if there is a mismatch between peers.

This is an interoperability concern. If the intent is to only support legacy checksum, this should probably be documented in the release notes/version description document. If the intent is for full compliance with CCSDS book 727.0-B-5, then this is missing.

Interim recommendation for FSW is to at least check this field on receipt of a Metadata PDU, and reject if the checksum type field is set to anything other than '0'.