Adjust import workflow configuration to support S3

[jimmymathews]

• Allows input_path, for the import workflow, to be an S3 URI.
• Adds some checks on the URIs to make sure the correct files are available.
• Adds some logic to make sure Nextflow will normally be able to access the appropriate credentials to pull from S3.

The result is that the Nextflow procedure to import a curated dataset into the database pretty much works as-is when the source files come from a folder in an S3 bucket, rather than from a local directory.

Note that Nextflow does support S3 "out of the box", but a subtlety is that it does not support the session-specific tokens that we have been using in the shell environment. Instead, in the session-specific case one must use the credentials from a user profile, and one must not allow Nextflow to try guessing credentials environment variables, because it will fail with a misleading error.

• There is also a slight adjustment to the metadata field inference, to allow some channels to be indicated as "computationally generated" without affecting the primary channel annotation.