Regression Script Refactoring

# Changes
### General
* Updated many locations that used make_path to use TestData's methods and attributes instead
* Stripped excess whitespace off the end of many lines to prevent potential future issues (python can be picky about whitespace)
* These updates have been tested in all modes (single-file, config-file, rebuild)

### Documentation
* Added google-style docstrings to nearly all classes and functions
* Changed existing documentation to google-style docstrings

### Constants
* Filenames consistent between gold and output directories are now constants

### TestData
* Changed constructor to also take an image name. The initialization of the TestData attributes is now in the constructor as opposed to a function in TestRunner
* Stores a DiffResults instance which represents the results of the database comparisons. This allows the reporting class to query TestData for the results instead of needing an instance of TskDBDiff [AUT-1062]
* Added additional attributes and methods for accessing files and directories in the output folder, reducing the use of make_path [AUT-1063]

### TestConfiguration
* Removed get_image_name as a method and made it a global function instead. It is a helper function that is used elsewhere in the script and should not be a member of TestConfiguration

### TskDBDiff
* Deleted the clear method as it is never called or needed
* Moved several functions needed by the Reports class to get the diff results to DiffResults because Reports now uses TestData's DiffResults instead of TskDBDiff
* Renamed run_basic_comparison to run_diff to reflect the outline of the regression testing redesign

### TestRunner
* Replaced the _generate_test_data function with a simple list comprehension