Add a function to symmetrize a LocalMatrix
When Seldon was used to handle local linear algebra, it provided an handy matrix class for symmetric matrices: only the upper half of the matrix was in fact stored in memory, and operations were overloaded knowing the represented matrix was symmetric.
To my knowledge, there is no such thing in Xtensor. However, some \a LocalOperator were written with the idea of filling only the upper half.
Current function is then to be called at the end of such symmetric operators: it will copy the values in the lower half of the matrix (in debug mode, there is a check the lower half is properly populated with zeroes).