Fix MdtReadoutElement::fillCache() for BMG chambers

Hi,

this MR brings two changes:

• upgrade the m_cachingFlag member variable of MuonDetectorTool to a property CachingFlag such that we can set it from outside and putting its default value back to 1, i.e. reverting !27956 (merged) (cf. ATLASRECTS-5196). So ATLASRECTS-5196 can be closed for now.
• fix a missing check for BMG chambers inside MdtReadoutElement::fillCache() since those chambers have cutouts for alignment parts which means not all tube numbers are present and non-existing tubes need to be skipped when calling the surface(tl, tube) method. Obviously, when turning on MuonDetectorTool.FillCacheInitTime, we still need to check how much this increases the memory consumption since we now initialize all MDT ReadoutElements in initialize(), no matter if they have hits or not.

This MR furthermore allows to run python -m MuonConfig.MuonSegmentFindingConfig --run --threads=8 -o out.root with more than one thread.

Adding @emoyse, @rosati for muon software

Adding @sroe, @wlampl for informing them

BTW: Jochen should receive at least some drink for that 😉