Fileformats

* Adds TypedSet generic class
* Renames BaseDirectory -> Directory and Directory -> DirectoryContaining
* Adds SetOf
* Adds fileformats.misc.Dicom class

This PR contains minor bug fixes around handling of file extensions

Modified the WithSideCars mixin so that it can support multiple side-cars

* Adds the WithQualifiers mixin class

By adding the WithQualifiers mixin, classes can be subtyped using class getitem, e.g. Zip[Png] represents a zip file containing a PNG image. Qualifiers can be interpreted differently depending on what makes sense for the qualified class. For example, Directory[Png, Gif] describes a directory containing PNG and GIF files, where as Xml[Svg] is an XML file that corresponds to the SVG format.

* Adds field types for use in typing strings, floats, bools and arrays
* streamlines the validation so both the required properties and the "checks" are tested at initialisation time

A small release, with a more descriptive warning messages on converter import fail