revid/config/parameter: add Default method to Parameter interface

[Saxon Milton]

We currently default parameters in the case that they are not set or have bad values. It would be handy to give the Parameter interface a Default() method so that if the Set() method returns an error, we can default the parameter (and probably return a soft error.) This is a prerequisite to simplifying the sections of code dealing with config update, validation and generation of DataTypes map for netreceiver/vidgrind.

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[Saxon Milton]

Config refactor needs whiteboard discussion between Saxon, Alan and Dan, which is not possible right now due to COVID. This is on hold.