Split Directory class

Define a base **Directory** class with get_files and filter, then make separate **report classes** that inherit from it

The current directory class is too big

Proposed usage:

```py
from file_processing import MetadataReport, AnalyticsReport, SimilarityReport

MetadataReport('path/to/dir', **kwargs)
AnalyticsReport(...)
...
```

Proposed implementation

```py
class Directory()
# init
# get_files
# filter
```

```py
class MetadataReport(Directory)
# methods
```

Note: Tests would need minor refactoring