Rename isConfigured() method in Service.hpp to allRegistersValid()

[ClemensElflein]

isConfigured() is misleading in the sense that it will return true BEFORE calling Configure(), because it just tracks if the registers are set (so that we can make sure at Configure() time, the registers are valid)

[rovo89]

Similarly, Configure() is actually more a CheckConfiguration(). All examples I've seen so far looked at the values of registers and returned false if they're e.g. out of bounds.