Add getter for RD53A/B chip ID

In some cases having a method directly from the FrontEnd instance to query it's ID is useful. Some methods, e.g. sendClear, take as input the chipID parameter which is only accessible via a protected data member of the Rd53a/bCfg instances. If you are calling these methods directly (as opposed to them being called within class methods that have access to m_chipID) then you need unambiguous access to the current chip's ID.

There is already the setter method, so this just adds a getter.