STYLE: include scalar.H instead of floatScalar.H/doubleScalar.H separately

applications/test/DiagTensor/Test-DiagTensor.C

applications/test/SphericalTensor/Test-SphericalTensor.C

applications/test/SphericalTensor2D/Test-SphericalTensor2D.C

applications/test/SymmTensor/Test-SymmTensor.C

applications/test/SymmTensor2D/Test-SymmTensor2D.C

applications/test/Tensor/Test-Tensor.C

applications/test/Tensor2D/Test-Tensor2D.C

src/OpenFOAM/db/error/error.H

src/OpenFOAM/db/error/errorManip.H

src/OpenFOAM/db/error/messageStream.H