Rename NewDataSet to DataSet

It was named this during the refactor. It is no longer the 'new' data
set class, it is 'the' data set class.
Merge pull request #328 from alphagov/new-data-set

Rename NewDataSet to DataSet
Attempt to convert excel floats to int

This matches the CSV parser and avoids failures against the JSON schema
for transactions by channel.
Merge pull request #329 from alphagov/convert-excel-float-to-int

Attempt to convert excel floats to int