ENH: finiteArea: modernise and clean up code

src/finiteArea/faMatrices/faMatrix/faMatrix.C

src/finiteArea/faMesh/faMeshDemandDrivenData.C

...convectionSchemes/faConvectionScheme/faConvectionScheme.H

...Schemes/gaussFaConvectionScheme/gaussFaConvectionScheme.C

...Schemes/gaussFaConvectionScheme/gaussFaConvectionScheme.H

...Area/d2dt2Schemes/EulerFaD2dt2Scheme/EulerFaD2dt2Scheme.C

...Area/d2dt2Schemes/EulerFaD2dt2Scheme/EulerFaD2dt2Scheme.H

...rea/finiteArea/d2dt2Schemes/faD2dt2Scheme/faD2dt2Scheme.C

...rea/finiteArea/d2dt2Schemes/faD2dt2Scheme/faD2dt2Scheme.H

...finiteArea/ddtSchemes/EulerFaDdtScheme/EulerFaDdtScheme.C

...Area/ddtSchemes/backwardFaDdtScheme/backwardFaDdtScheme.C

...s/boundedBackwardFaDdtScheme/boundedBackwardFaDdtScheme.C

...s/boundedBackwardFaDdtScheme/boundedBackwardFaDdtScheme.H

...initeArea/finiteArea/ddtSchemes/faDdtScheme/faDdtScheme.H

...dtSchemes/steadyStateFaDdtScheme/steadyStateFaDdtScheme.C

...initeArea/finiteArea/divSchemes/faDivScheme/faDivScheme.H

src/finiteArea/finiteArea/fac/facAverage.C

src/finiteArea/finiteArea/fac/facAverage.H

src/finiteArea/finiteArea/fac/facDdt.H

src/finiteArea/finiteArea/fac/facDiv.H