pixel ID scheme for ITk

XML-based customization of pixel ID readout.

Fixes ATLITKSW-72 for Step3 layout (InclinedAlternative), in combination with ITKLayouts/ATLAS-P2-ITK-17-08-00 branch. Uses generic XML names (layout to be specified via geometry tag, or ITKLayouts branch)

Redefines b-layer for SLHC layouts (see below). PixelID::is_blayer(id) becomes obsolete and will be replaced with PixelID::is_innermost(id). A warning is thrown in order to identify clients.

Main idea : dissociate the GeoModel layout structure and the readout identification by redefining the readout layers via xml.

Limitation: can only work by splitting GeoModel layer into sublayers, i.e the pixel inner endcap is defined as single GeoModel layer and the readout is customized.

Gains : fixes numerous problems accompanying LTF2 developments (overflowing id ranges, hardcoded offsets, mismatched material for pixel services).

Options: this import contains the redefinition of b-layer for SLHC layouts (= barrel AND endcap layer 0). Another possibility would be to redefine part of GeoModel endcap layer as readout barrel region (using the same xml customization) but this would need an update of SiNumerology class and its clients.