remove maximum number of channels limitation (#340)

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.

Author: daniestevez

sigmf-schema.json

@@ -83,7 +83,7 @@
           "description": "Number of interleaved channels in the Dataset file, if omitted this is implied to be 1, for multiple channels of IQ data, it is RECOMMENDED to use SigMF Collections instead of num_channels for widest application support.",
           "default": 1,
           "minimum": 1,
-          "maximum": 1000,
+          "maximum": 9223372036854775807,
           "type": "integer"
         },
         "core:offset": {