Addresses Issue #117 Add metagenome confidence intervals

The current implementation incorporates uncertainty due to errors in genome prediction, and uses that uncertainty to generate 95% confidence intervals and variances for the metagenome predictions.   

NOTE:  due to a mismatch in the current version of the pre-calc'd count table and variance table I could only do testing at the script level and on a small synthetic example.   I'm raising an issue for this, but before 1.0 we need to fix the precalc'd files and do a biological test to benchmark speed.

Changes:
   -- new variance prediction code
   -- new test code 
   -- abstracted out the script code for file IO so it could be reused for variance input/ouput as well as count tables.
      variance tables will use user-input Greengenes version in the same way as count tables, but expect the variance suffix before .biom.   
   -- Modified the user interface to allow for input of custom variance tables (parallel to how count tables currently work).
   -- added example inputs and outputs.   I noticed that some old example inputs and outputs were stalling out prior to starting work, but this is already a large PR, so I think we should address this as a separate issue.  For now added a custom ids example that runs rapidly.