A generator the size of a shoebox that could power a city.  That’s what David Grammer of Macon promised investors, and he convinced 84 of them to put up $3.3-million to develop the “Boydoplex” at laboratories in Hannibal and Evansville, Indiana.   Grammer was sentenced in federal court yesterday to 78 months in prison and ordered to pay $2.7-million in restitution on two counts of mail fraud.