merge: Support single inputs

[victorlin]

Context

augur merge currently requires at least two inputs in order to perform a merge on metadata or sequences.

augur/augur/merge.py

Lines 161 to 162 in c7fb169

	if not len(args.metadata) >= 2:
	raise AugurError(f"At least two metadata inputs are required for merging.")

This limitation makes sense in the context of merging. However, augur merge is used not only to merge but also decompress and add source columns. This is because it is more generally used as the entry point for input data in pathogen workflows.

Description

Support single inputs such as augur merge --metadata a=metadata.tsv. This would remove the need to separately handle:

1. decompression (nextstrain/pathogen-repo-guide@fed74be, Document input compression support zika#93)
2. presence of source columns for the proposed keep_all option