--[Bugfix] - Storage of attribute source directory paths

[jturner65]

Motivation and Context

This PR changes how attributes' directory paths are stored so that they end in an os separator ('/'). This corrects an issue where path merging was sometimes giving erroneous results in certain conditions, and also simplifies and unifies path building code called from the scene dastaset load.

How Has This Been Tested

Locally c++ and python tests.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.