lower/upper triangular matrices are defined and used transposed from math usage

[dougrm@…]

The first paragraph of http://www.boost.org/doc/libs/1_37_0/libs/numeric/ublas/doc/triangular.htm#triangular_matrix describes when elements of lower and upper triangular matrices are fixed at 0, The definition is transposed from what I have always understood to define lower/upper triangular. With the given definition, a lower triangular matrix has enforced zeros in the lower triangular portion (when i>j). To me, that is an upper triangular matrix. Oh, I just looked at a little bit of the code and I think it does what the document says -- i.e., the wrong thing. So, this ticket should apply to numeric/ublas software also.

[Gunter]

(In [50916]) * fix and close #2686

• replaced the word "upper" by "lower" because the description is for lower triangular matrices

• added more detailed example for the use of triangular matrices and adaptors