Removes sensor-related terms to fix issue 462

[turingtest37]

Fixes #462

[uscholdm]

• gist:MessageDefinition looks to be sensors-specific. I propose that we remove it unless there is justification for having it independently from sensors.
• gist:directs looks to be sensors-specific. I propose that we remove it unless there is justification for having it independently from sensors.
• gist:accepts looks to be sensors-specific. I propose that we remove it unless there is justification for having it independently from sensors.

[rjyounes]

DECISION:

• Remove MessageDefinition and directs
• Rewrite definition of accepts as "Relates something to the types of inputs it accepts."

[rjyounes]

See decision in main comment thread.

[rjyounes]

@uscholdm to contact @turingtest37 to resolve questions about term deletions.

[uscholdm]

Remove MessageDefinition