DetectorType enum to Design

ReadoutGeometryBase is now (or will be) used by several detectors, and so it seems useful to have an easy (optional) way to find out which detector an element belongs to without having to retrieve an IdHelper (and then potentially dynamic_cast to the appropriate type).

This is added via a new enum in the Design class, which can be optionally set to something other than its Undefined default value downstream by the derived design classes specialized for the different detectors. Current options (representing all the users of this base class that I'm aware of) are:

Undefined,PixelBarrel,PixelEndcap,PixelInclined,StripBarrel,StripEndcap,BCMPrime,PLR,HGTD

This can then be retrieved by element->design().type() - an initial use of this pattern is implemented in this MR in order to avoid WARNINGs for "inclined" modules in the ITkPixel.

Nomenclature, etc, can certainly be improved so comments are welcome.

cc @tlari @hhayward @tadej @ncalace @tstreble @dshope