Check units of data before training a model

[tk-king]

• [ ] When training a model, first check that all used time-series have the same units.
• [ ] If there is no unit set for a timeseries, ask the user to set one (with offset and scale to bring the data into the correct representation)
• [ ] When time-series with the same name but different units exist, ask the user to convert the units first, so that all time-series have the same units.