Added new implementation to Identity tensor

[miguelmaso]

Description
Changes in TensorAlgebra. Identity tensors are exported as variables I2, I3..., instead of functions I2(), I3()...

A private function to construct the identity without hardcoded values has been added:

const I_(N) = TensorValue{N,N,Float64}(ntuple(α -> begin
  i,j = _full_idx2(α,N)
  i==j ? 1.0 : 0.0
end,N*N))

This function is compile-time thanks to ntuple. It can be checked with @benchamrk that both implementations are equivalent, see the TensorAlgebra tests.

All the required replacements in PhysicalModels have been done.

Its better to wait #3 to be merged and use the CI.