error nicely on missing --process-obs-metadata

[antgonza]

If you run: biom convert -i test.txt -o new_otu_table.biom --to-hdf5 --table-type="OTU table" you will get:

  File "biom-format/biom/table.py", line 3290, in vlen_list_of_str_formatter
    data[i, :len(value)] = value
TypeError: len() of unsized object1

but the solution is to pass --process-obs-metadata taxonomy so the command will look like: biom convert -i test.txt -o new_otu_table.biom --to-hdf5 --table-type="OTU table" --process-obs-metadata taxonomy

[wasade]

I think this is a bug in Table.to_hdf5, @josenavas, what are your thoughts here?

[josenavas]

I think we can specifically check for the type of the taxonomy metadata category, and raise a useful error.