Add getters for eta/phi indices in format used by L1Topo

This update adds 2 getters to xAOD::eFexEMRoI_v1, which provide eta and phi indices with the ranges and granularities used by, and requested by, L1Topo. It also changes the range of the existing iEta() getter (integer eta index at tower granularity) to use a consistent convention (signed). The data content of the object is not changed, so it has no implications for persistency.