speed up GetExp

In interpolation, the number of points is usually huge. GetExp(const Array< OneD, const NekDouble > & gloCoord) spends a lot of time on finding the correct element.

In most applications, the successive points are not far from each other. So can we introduce a cache mechanism in GetExp and remember the last element index as a initial guess for the next point?

I did a initial test on Lagrange points tracing. When the number of points is very large, the cache method achieves speed up by several tens of times.