Fix boundaries for TENSOR_DIFFUSIVITY model

[rmcdermo]

This crashes in debug mode because boundary arrays for velocity gradient have never been handled properly.

At line 1650 of file ../../Source/turb.f90
Fortran runtime error: Index '-1' of dimension 1 of array 'uu' below lower bound of 0

FM_15cm_Burner_C2H4_20p9_4cm.fds.txt