Improvements to foamToEnsight*

During testing of issue #231 (closed) noticed that startup/conversion time (per step) to be dependent on the number of times-steps converted (foamToEnsight). The per step conversion time for foamToEnsightParts is constant. In serial, the writing times for foamToEnsight are generally much slower than foamToEnsightParts. No comparison in parallel, since foamToEnsightParts is not yet parallelized.