added failing NaN test
fixed check_symmetric so it throws for NaNs not on the main diagonal
added doc in the code for check_symmetric
added failing nan tests for check_pos_semidefinite
added nan check to check_pos_semidefinite and added doc in code
fixed check_pos_semidefinite so it works with 1x1 nan matrix
Merge branch 'feature/issue-891-check-pos-semidefinite-nan' into feature/issue-891-check-spsd-matrix-nan
added NaN test for check_spsd_matrix
added doc in code for check_spsd_matrix