Add timeout for large directory parsing in source processing

When sending large directory paths to CytoTable, it could take time to process these which manifests without warning. These could be directories which are submitted in error, meaning returning a warning could be helpful for the user to understand better that they may need to double check their source path.

Start by locating the source-processing entry point that parses directory paths and review how long-running parsing is currently reported. Define the timeout and warning behavior for unusually large directories, then add coverage showing that users are warned when processing exceeds the chosen limit.

Written by the indexing model from the issue text.