ENH: add fieldLevel handling for surface writers (#2382)

- this can be used to apply a uniform field level to remove from
  a sampled field. For example,

      fieldLevel
      {
          "p.*"   1e5;        // Absolute -> gauge [Pa]
          T       273.15;     // [K] -> [C]
          U       #eval{ 10/sqrt(3) };  // Uniform mag(U)=10
      }

  After the fieldLevel has been removed, any fieldScale is applied.
  For example

      fieldScale
      {
          "p.*"   0.01;       // [Pa] -> [mbar]
      }

  The fieldLevel for vector and tensor fields may still need some
  further refinement.