* Simplifications and modularity for the `Disk` entry class
* Implements a first tentative of support for BTRFS
* Removes some unnecessary `,` in configuration testing module
* Inverts two testing suite configuration entries to bypass PyLint R0801
* `Disk` entry actually shows values in Gibibytes (not Gigabytes)
* Allows `df` to fail on BTRFS (and RAM-based FS) only systems
* Increases precision of `Disk` value by adding decimals