Add validation for FolderData, RemoteData and SinglefileData type inputs

[borellim]

We might want to do some simple checks on the contents on folders and files passed as input, to quickly give a feedback to the user without waiting for the actual calculation to start and crash.

Questions:

• What level of validation do we want?
• How do we access files in a RemoteData node?
• How do we avoid rejecting inputs that would actually be accepted by the code? How do we do this for all versions of the code?

Originally raised by @odarbelaeze and @sphuber .