Modify THcHallCSpectrometer

    THcHallCSpectrometer inherits from THaSpectrometer which has method
    GetPcentral and a member fPcentral.
     THcHallCSpectrometer had been using fPCentral but decided to
    use fPcentral.
    Delete fPCentral in THcHallCSpectrometer.h
    Replace all fPCentral  by fPcentral in ReadDatabase and FindVertices methods.