Make precision adaptors modifiable

Currently the precision adaptors are fixed at construction time. However, if we use refPtr internally, we can make them adjustable after construction.

Eg,

labelList dummyAdj;
labelList dummyXAdj;

ConstPrecisionAdaptor<SCOTCH_Num, label, List> adjncy_param(adjncy);
if (adjcny.empty())
{
    dummyAdj.resize(1, Zero);
    dummyXAdj.resize(1, Zero);

    adjncy_param.set(dummyAdj);
    ...
}