[CMAKE] Add option to xTensor and xMapping creation
xTensor and xMapping library were not managed by any options. For sure both may/are (I don't know) needed by almost all other libraries. But it is not the question of "need" but the question of "when" that those new option answer to. In many free project the moto to get them in use is "configure/cmake, make, make install" Here if we consider that all ours repositories works like that, xFiles and xTest and ...(Applis) may be compiled separately with steps: "make install" Introducing those options gives a way to avoid xMapping and xTensor compilation all the time. They are compiled and installed when dealing with xFiles. Then there is no need to compile then for XTest .... Actual vision is to use the same GEMUtil CMakelist.txt and switch option. That introduce this constraint but maybe in future we will split that file in some peaces : option block, Xfiles, xTest, APPLI.