mutate.traitdata()

[fdschneider]

I composed the function mutate.traitdata() to modify a traitdata object and add derived traits.
Still open is the derivation of units for these traits: this requires units being given in the first place.

Since units can be provided in the trait standardization procedure, this is of minor importance.

This replaces issue #11.

[fdschneider]

The problem with this function is that it cannot combine further measurement level information, such as data source, aggregate statistics, units etc. into new content.
(It might be possible to provide this for units though)