#657 Validate input files from config file

[emontag47]

Description of proposed changes

What is the goal of this pull request? What does this pull request change?
This request attempts to fix issue 657 and validate the input files from the config file by making sure the file names do not have spaces in them and exist in the given directory. It raises an error and prints what went wrong if the file name is not valid.

Related issue(s)

Fixes #
Related to #657

Testing

What steps should be taken to test the changes you've proposed?
Test to see if the program exits correctly and prints a descriptive error message when an input file with an invalid name is used.

If you added or changed behavior in the codebase, did you update the tests, or do you need help with this?
Yes, my partner and I changed the codebase but did not updated the tests. We would need help with this.

Release checklist

If this pull request introduces backward incompatible changes, complete the following steps for a new release of the workflow:

• Determine the version number for the new release by incrementing the most recent release (e.g., "v2" from "v1").
• Update docs/src/reference/change_log.md in this pull request to document these changes and the new version number.
• After merging, create a new GitHub release with the new version number as the tag and release title.

If this pull request introduces new features, complete the following steps:

• Update docs/src/reference/change_log.md in this pull request to document these changes by the date they were added.