Wrap HMat library into OpenTURNS and use it in TemporalNormalProcess

New USE_HMAT option in CMakeLists.txt, which is OFF until source code is released, which should happen very soon now.
Replace TemporalNormalProcess-UseGibbsSampler resource key by TemporalNormalProcess-SamplingMethod, which can take value 0 (Cholesky), 1 (HMat, if available) or 2 (Gibbs).
New CovarianceHMatrix class.

distcheck ok

git-svn-id: https://svn.openturns.org/openturns/branches/hmatrix@4136 59136d6a-6dff-0310-91ca-bfc5db6e1932