fieldToCell does not use lookup; always reads from disk

Functionality to add/problem to solve

fieldToCell currently always reads from disk. Hence it cannot be used in e.g. a functionObject to select some cells during the run (). Would be nice if it would try lookup first instead.

Proposal

• add database lookup first and loading from disk only if that fails
• move fieldToCell to finiteVolume and use proper fields

Example of desired use: plot all cells where the pressure is between 5 and 1000.

    vtkWrite1
    {
        type            vtkWrite;
        libs            ("libutilityFunctionObjects.so");
        timeStart        10;
        writeControl    timeStep;
        writeInterval   1;
        format          binary;
        legacy          false;
        decompose       false;
        fields          (p U);

        selection
        {
            threshold
            {
                action  use;
                source  fieldToCell;
                field   p;
                min     5;
                max     1000;
            }
        }
    }

There is currently also a bug in that it searches for the last valid p,U instead of loading them from the current time (and failing).