Modify inputs in rclone check and checksum

[delfiterradas]

Closes nf-core/datasync#52:

• Accept rclone cofig
• Set inputs to be val instead of path, to avoid staging of files
• Capture exit codes
• Remove empty files from output

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[itrujnara]

Most of the changes are fine, just some small comments to consider

[itrujnara]

Is this test not meaningful anymore?

[delfiterradas]

This won't work anymore since now that the files are not staged it only accepts directories, not files. Also it won't be able to list the files in https://raw.githubusercontent.com directories. This scenario is now included in "test - md5 - differs".