Neil Rossen, CFO of Presstek Inc., will resign from the company effective Dec. 31. Presstek is initiating a search for a new CFO.   . . .   ART Advanced Research Technologies Inc., developer of laser and thermal imaging technologies, recently reorganized its medical and industrial divisions.   . . .   Intevac said it's reducing its workforce by 21 employees, to 185, which will cut operating and manufacturing costs by about $1.6 million per year. The company said it has also reduced management salaries and will close for two weeks during the fourth quarter.