Revised DataStreams
Fix similar
Clean up unary operators
Clean up matrix multiplication

May need additional tests
Only define the necessary similar operator, and make it accept types
Clean up comparison operators
Clean up arithmetic operators
Clean up most remaining operators, and restore operator lists for now
Remove cov_spearman and colffts from tests
Move all macros to top
Make operator arrays const
Fix ambiguity warnings
Fix f(::AbstractArray, ::AbstractDataArray) for binary operators
Fix pairwise operators when first element fo a DataVector is NA
Add tests for PooledDataArray and pairwise operators
Merge pull request #351 from simonster/operators

RFC: Clean up operators