Tightened BurnishSpecifier initialization.

BurnishSpecifier must now set either ...

    burnish_each_division=True

... or ...

    burnish_output=True

... at initialization.

NOTE: this should already be the case in all actual use of BurnishSpecifier.
(Because a burnish specifier without either of these values set to true is
ineffectual.) This commit simply enforces the fact.