Add argument 'template' to standardize()

[fdschneider]

The output created by the standardize functions is parsimonious, i.e. contains only the columns that have been explicitly provided. A template argument should provide the terms and order of terms that are desired as output. This would be used to create a standardized output, e.g. for upload to BExIS or other services that expect a particular structure.

The template could be just a vector of exact column names (from the vocabulary), or a named vector that renames columns according to the desired output. If wrapping around transform, this could even provide new computed columns.

This functionality adds quite generous power to the package, since it allows to map any input onto any output.