clang-6 warnings

[yurivict]

Getting these on FreeBSD. The warnings about 'control may reach end of non-void function' could be potentially dangerous.

./SampleStepper.h:59:39: warning: control reaches end of non-void function [-Wreturn-type]
  virtual EnergyFunctional& ef(void) {}
                                      ^
./SampleStepper.h:60:36: warning: control reaches end of non-void function [-Wreturn-type]
  virtual ChargeDensity& cd(void) {}
                                   ^
In file included from ./BOSampleStepper.h:22:
./SampleStepper.h:59:39: warning: control reaches end of non-void function [-Wreturn-type]
  virtual EnergyFunctional& ef(void) {}
                                      ^
./SampleStepper.h:60:36: warning: control reaches end of non-void function [-Wreturn-type]
  virtual ChargeDensity& cd(void) {}
                                   ^
In file included from WavefunctionHandler.C:19:
./WavefunctionHandler.h:46:7: warning: private field 'read_from_gfdata' is not used [-Wunused-private-field]
  int read_from_gfdata;
      ^
In file included from RSHFunctional.C:37:
./RSHFunctional.h:42:16: warning: private field 'alpha_RSH_' is not used [-Wunused-private-field]
  const double alpha_RSH_, beta_RSH_, mu_RSH_;
               ^
In file included from XCOperator.C:18:
./XCOperator.h:35:24: warning: private field 'cd_' is not used [-Wunused-private-field]
  const ChargeDensity& cd_;
                       ^
XCPotential.C:181:24: warning: argument type 'double (*)[2]' doesn't match specified 'MPI' type tag that requires 'double *' [-Wtype-safety]
    MPI_Allreduce(&sum,&tsum,2,MPI_DOUBLE,MPI_SUM,vbasis_.comm());
                       ^~~~~   ~~~~~~~~~~
XCPotential.C:181:19: warning: argument type 'double (*)[2]' doesn't match specified 'MPI' type tag that requires 'double *' [-Wtype-safety]
    MPI_Allreduce(&sum,&tsum,2,MPI_DOUBLE,MPI_SUM,vbasis_.comm());
                  ^~~~         ~~~~~~~~~~
XCPotential.C:362:24: warning: argument type 'double (*)[2]' doesn't match specified 'MPI' type tag that requires 'double *' [-Wtype-safety]
    MPI_Allreduce(&sum,&tsum,2,MPI_DOUBLE,MPI_SUM,vbasis_.comm());
                       ^~~~~   ~~~~~~~~~~
XCPotential.C:362:19: warning: argument type 'double (*)[2]' doesn't match specified 'MPI' type tag that requires 'double *' [-Wtype-safety]
    MPI_Allreduce(&sum,&tsum,2,MPI_DOUBLE,MPI_SUM,vbasis_.comm());
                  ^~~~         ~~~~~~~~~~
Matrix.C:300:19: warning: 'zdotc_' has C-linkage specified, but returns user-defined type 'complex<double>' which is incompatible with C [-Wreturn-type-c-linkage]
  complex<double> zdotc(const int *, const complex<double>*, const int *,
                  ^
Matrix.C:90:20: note: expanded from macro 'zdotc'
#define zdotc      zdotc_
                   ^
Matrix.C:302:19: warning: 'zdotu_' has C-linkage specified, but returns user-defined type 'complex<double>' which is incompatible with C [-Wreturn-type-c-linkage]
  complex<double> zdotu(const int *, const complex<double>*, const int *,
                  ^
Matrix.C:89:20: note: expanded from macro 'zdotu'
#define zdotu      zdotu_
                   ^
Matrix.C:2028:1: warning: control may reach end of non-void function [-Wreturn-type]
}
^
Matrix.C:2044:1: warning: control may reach end of non-void function [-Wreturn-type]
}
^
Matrix.C:2134:1: warning: control may reach end of non-void function [-Wreturn-type]
}
^
Matrix.C:2150:1: warning: control may reach end of non-void function [-Wreturn-type]
}
^