Piksi: Add mask satellite message.

[gsmcmullin]

I can't rebuild the latex documentation, but this fails on master too.

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[mookerji]

Wouldn't you actually want this to be something like:

        - mask:
            type: u8
            desc: Mask of systems that should ignore this satellite.
            fields:
                - 0:
                    desc: Acquisition channel
                    values:
                        - 0: Enabled
                        - 1: Skip this satellite on future acquisitions
                - 1:
                    desc: Tracking channels
                    values:
                        - 0: Enabled
                        - 1: Drop this PRN if currently tracking

As example:
https://github.com/gsmcmullin/libsbp/blob/mask_satellite/spec/yaml/swiftnav/sbp/navigation.yaml#L194

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[cbeighley]

lgtm