ENH: the adjoint grid displacement field (ma)

is now appended by the name of the adjoint solver, if more than one
exist. This was necessary for an accurate continuation since, before
these changes, only the ma field of the last solver was written. As a
result, when restarting the first adjoint solver was reading the ma
field of the last one. No changes are needed in fvSolution and fvSchemes
w.r.t. the previous code version.