ASIdiSPIM: changes to slice timing calculations; fixes problem seen with Flash4 due to not accounting for delay between trigger and exposure start, also shaves 0.25ms from minimum slice period in easy timing mode

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14766 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd