add option checkpoint_every_divisor

this patch adds an option checkpoint_every_divisor analogous to the similarly named option present for periodic output. It triggers a checkpoint whenever cctk_iteration % checkpoint_every_divisor == 0 rather than (cctk_iteration - iteration_of_last_checkpoint) >= checkpoint_every which can be useful if one needs to trigger checkpointing eg via HTTPD.

Keyword: CarpetIOHDF5