Follow simplification of IElectron interface + misc. cleanup

• adapt IElectron implementations to simpler interface
• remove commented out obsolete configurations
• prefer extends over virtual inheritance
• use enumerate in a few places to simplify things
• move SpreadEstimator into the single .cpp file which uses it
• prefer ToolHandle in CaloFutureElectron
• prefer STL algorithms over raw for loops
• do not assume all cluster observables are double
• remove NeutralProtoParticleAddNeutralID, as this uses a tool with the INeutralIDTool interface, but there is no implementation of this interface -- hence this algorithm can not have been running in any (current) configuration...
• remove unused ChargedProtoParticleAdd... algorithms

must be applied in conjunction with LHCb!2782 (merged)