remove implicit conversion of Cell to Iterable

pollute method space, and is surplus to requirement as the relevant methods have been implemented explicitly
Add test for Series.++
Remove unneeded s
Add tests for Series.closestKeyTo
Improve coverage for Series.zipMap
Merge pull request #33 from mrvisser/topic/series-merge-tests

Add test for Series.++
Merge pull request #34 from mrvisser/topic/series-closestkeyto-tests

Add tests for Series.closestKeyTo
Merge pull request #28 from dwhjames/topic/remove-cell-to-iterable

remove implicit conversion of Cell to Iterable