sbp2rinex does not generate valid rinex when using toJson() method from SBPMessage.java

[olvlucas]

When receiving SBP messages using libsbp java client and then printing the JSON of received SbpMessages to a file, it is impossible to use sbp2rinex on this file. This happens because the toJson method does not include the CRC field on the output being impossible to generate a valid rinex.

[silverjam]

For a more long term solution the SBPHandler class should populate a CRC field in the SBPMessage class, but if making the CRC16 class and methods public works around this issue, then that is fine in the interim.

[silverjam]

Closing since we merged the requested change. Please re-open if the more permanent solution is needed.