Abstract out default filename handling into its own functions.  This allows reuse for loading tables of variances
Tests for variance combination fns
Added variance combination fns
Fix a couple of typos for metagenome prediction tutorial.
Improve error handling when given bad OTU table

Fix #133
Raise a ValueError with informative message if given OTU table
doesn't have any overlapping OTUs with precalculated file.
Working version of predict_metagenomes_with_variance
Test code for metagenome prediction treating OTUs as scalars
Implementation of metagenomic variance propagation that passes test code.  Output code still required
Added support for writing tables of variances and confdience intervals, and fixed an issue with the location of the precalculated variance table
Fixed output, did a full script test on the predict_traits.py example
Fixed outputs for upper and lower CIs, which were switched
Cleanup before issuing pull request
Custom trait table for usage example.  Uses new-style tab-delimited predict_traits.py output
Custom trait table variances for usage example.  Uses new-style tab-delimited predict_traits.py output
OTU table for custom ids usage example.
metagenome prediction output for the custom trait table usage example.
Variances for the custom ids example
Upper CI 95 for the custom id example
Lower CI 95 for the custom id example
Added usage example that utilizes custom ids and new-style predict_traits.py tab-delimited output.