Heatmap normalise_by only works by sample names

[bernt-matthias]

According to the docs I can

A variable or a specific sample in the metadata to normalise the counts by.

If I do

amp_heatmap(AalborgWWTPs, normalise=T, normalise_by = 'Plant')

I get: Error: Plant is not found among group names, cannot normalise

I debugged this a bit, and it seems that normalisation is only possible by a sample at the moment.

Note, For me it would be fine to fix the docs.