xVector, xTensor2* and xTensor4 are now template classes
xTensor4Isotropic, xTensor4AnisoPlaneStrain and xTensor4AnisoPlaneStress
remain double-valued objects as they make sense only in this context.

By default, value type is double, so that you may only have to replace
xVector by xVector<> and so on in your code.

Update Tensor operations such that xTranspose, xVonMisesNorm etc...
Theya are also double-valued by default : xTranspose<> is equivalent to
xTranspose<double>

Update the rest of the repository accordingly