updates to allow core_diversity_analyses to run in 'print only' mode

'print only' mode allows the user to generate the list of commands that would be run by the workflow, without actually running them. this is really useful for recovering from failed runs.

- added parameter ``suppress_md5`` which allows the caller to turn off computation of md5 sums when generating the log file. this is redundant when these functions are called from core_diversity_analyses as core_diversity_analyses itself computes and logs the input md5sums. this additionally caused an issue because the low-sequence-count sample filtering step, which creates the otu table that gets passed to the individual workflows, isn't created when these functions are called in 'print only' mode. trying to compute the md5 of that file would therefore fail. we don't really need the md5 of that file however as the file itself is included in the output (and linked from the index.html page).
- removed some checking for the the existence of the input otu table. this was inconsistently done through the different workflows, but isn't really necessary as it now always happens during parsing of the command line parameters.
added support for running core_diversity_analyses in 'print only' mode

this allows users to print the list of commands to be run, but not actually run them.
all links now open in new tab/window
added suppress_md5 option to these workflow functions

for consistency with the 'downstream' workflows