NEW: adds ImmutableMetadata semantic type and supporting formats

[gregcaporaso]

Still to-do:

• unit tests
• should the formats be called MetadataFormat and MetadataDirectoryFormat (i.e., drop the Immutable from the beginning of both, since there is nothing about the formats that differs from our regular metadata files)? looking for input on this

fixes #292

A little example of how this can be used:

$ qiime tools import \
  --type ImmutableMetadata \
  --input-format ImmutableMetadataFormat \
  --input-path sample-metadata.tsv \
  --output-path sample-metadata.qza
$ qiime metadata tabulate \
  --m-input-file sample-metadata.qza \
  --o-visualization sample-metadata.qzv
$ qiime tools export \
  --input-path sample-metadata.qza \
  --output-path md/

[lizgehret]

Generally this all looks good @gregcaporaso! In terms of your question about whether this should be named ImmutableMetadata vs Metadata - I am leaning towards ImmutableMetadata just because that is a bit clearer in the intention behind adding this type and associated formats. I think that would also help folks distinguish between the Metadata class within QIIME 2 🤷‍♀️

[gregcaporaso]

That makes sense to me @lizgehret! I think we can merge this one when you're ready.