fix timestamps after gaps in SRM files

"secs" is miscalculated for gaps between blocks: It takes the delta
between last chunks *END*time and next chunks start time - and thereby is
off by RecInt. This error adds up for each block/gap - so, the n'th block
will by off by n seconds.

fixes issue #582.
Merge pull request #861 from rclasen/issue-582

fix timestamps after gaps in SRM files