Version to 0.3-1 to distinguish from release
Refactor column handling

 - allow arm-based data to be either [mean, std.dev, sampleSize] or
   [mean, std.err] for consistency with relative-effect data.
 - let mtc.model add an std.err column to the data before calling any
   of the code/data generating methods.
 - have each likelihood/link require its own set of columns, in
   preparation of likelihoods that require additional information
 - no longer test presence of columns on reading mtc.network, since only
   the likelihood/link know which columns are required
Test likelihood/links are fully implemented
Implement poisson/log inference
Add poisson/log integration test
Remove redundant fields from model data