Dataset.views() works for non-slices
Refactored Dataset

The goal is to move Dataset toward being a simple container of variables,
dimensions and attributes.
Math with Variables objects

Including broadcasting by dimension names!
Refinements to Variable math

Includes edits in response to Alex's comments on my PR.
__repr__ for Dataset and Variable
moved source directory from polyglot to scidata
renamed module package to scidata
Merge pull request #11 from akleeman/variable-math

Math with Variable objects