Add command line switch --keep-file-list to not delete given file list after indexing.

Implement (correctly) help text accessible via --help.

Refactor to allow for future support of various file types in addition to netcdf.

No additional features were implemented.

• Fixed sql syntax errors in a few prepared statements.
• Added unit tests for some classes.
• Updated code docs.

• Add --dry-run option.
• Fix compiler warning.

Add new columns to Variables table: long_name, valid_min/max.