LduMatrix: coupled solvers are now run-time selectable for solving fvMatrices by conversion of the fvMatrix/lduMatrix to LduMatrix

This is so that surfaceFeatureExtract will compile without boost/mpfr

LduMatrix: Added support for both component-coupled and component-independent forms of PCG and PBiCG

read option

createShellMesh and introducing io::READ_IF_PRESENT option
in polyMesh constructor from components

Buoyant solvers: changed the sign of phig to be consistent with the sign of the buoyancy force on the RHS of UEqn

e.g. in transportProperties

phases (air water);