Add convert_to_gwp() to a dataset

[crdanielbusch]

Is your feature request related to a problem? Please describe.

We need to create a loop every time we want to convert the units for a data set.

Describe the solution you'd like

We would like to add an accessor for datasets for convert_to_gwp().

[JGuetschow]

This is not trivial as we might have variables that can't be converted, e.g. gas baskets. What is the use case you have in mind?

[mikapfl]

The use case for this is plotting a dataset which exists in mass units only so far.