added failing NaN test for check_pos_definite
fixed check_pos_definite for NaN values
added another test case for check_pos_definite
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 doc in code for check_pos_definite
added NaN test for check_cov_matrix
added doc in code for check_cov_matrix
Merge branch 'feature/issue-891-check-symmetric-nan' into feature/issue-891-check-cov-matri-nan