Use a message whitelist rather than mask

[henryhallam]

Per discussion, let's use a list of desired output messages rather than the mask system.

As well as allowing full customization, we should include a few sensible default lists, e.g. "Default navigation output", "Minimal baseline output", "Default base station to rover", "Minimum data base station to rover". These defaults could either live in Piksi or in Console. They could also be specified with some additional field, e.g. 'message_groups', in the sbp yaml.

cc @fnoble @cbeighley @denniszollo @mookerji

[fnoble]

Perhaps we could have the behavior that is the list is empty then it counts as let everything through?

[denniszollo]

What about frequency? Do we think users would want to specify the frequency in addition to what messages they receive? Especially for people with radio-constrained bandwidth, many applications may need to have some information at lower or customizable frequency. For instance, you might want your posistion outputs at 10hz, but the tracking message would be fine once every 10 seconds for the application.