Add option to output data from substeps (!112)

* add more colors to highlight in which files comparisons failed

* IntegrationTests: always use terminal coloring to highlight results

* update update_test script

* include substep outputting in some reftests

* teach IntegrationTests about substep_output.h5

* add unit test

* move substep savecallback logic into convert_to_remote_savecallback!

only add remote savecallback when requested

* enable substep output

* introduce AbstractCallbackTrigger and CallbackTrigger

* add parameters