Remove maximum number of channels limitation

[daniestevez]

In df53293, the SigMF spec was converted from Markdown to a JSON schema. Minimum and maximum values were added for many fields. In particular, the core:num_channels field got a maximum value of 1000. The Markdown specification did not specify a maximum value for core:num_channels, so in a sense this is a breaking change between v1.2.1 and v1.2.0 of the spec.

The limitation breaks some usage of SigMF to represent channelizer data, such as the output of the a polyphase-filterbank, or (power-detected) spectrogram data.

This removes the limitation by setting the max value to 2**63 - 1, which is the max value used for other fields.

[daniestevez]

I noticed this when some internal tooling that handles spectrogram data broke with a new release of sigmf-python. So I'm interested in how to get the fix through to sigmf-python. Since the JSON schema is copied in sigmf-python, it seems that the appropriate way to proceed is to wait for this PR to be merged, tag a release of SigMF, and then make a sigmf-python PR to copy the new JSON schema. Is that right?

[777arc]

Change makes sense! Yep that's the process, but we can make sure to do it quick in this case, in fact I'll merge and tag the patch release now