document which Matrix functions that destroy references/views to Matrix

[Peter]

For example:

Matrix X(3,3);
double& a = X(0,0);
X.resize(4,4);

It's obvious that 'a' is hanging, but for other operations like +=, *=, ::transpose() etc it's not obvious and needs to documented.

[Peter]

References, iterators and views are invalidated when gsl_matrix* m_ is reallocated or when blas_result_ is used and subsequently swapped with m_. That means (afaicu) that the following functionality imply invalidated references/iterators/views:

• ::resize (unless size is unchanged)
• ::transpose (unless square matrix)
• operator=(const Matrix&) (unless dimensions on lhs and rhs are the same)
• operator=(Matrix&&) (regardless of dimensions)
• operator=(const MatrixExpression?&) (regardless of dimensions)
• operator=(MatrixExpression?&&) (regardless of dimensions)
• operator*=(const Matrix&) (and documented)
• inverse_svd(2)
• nan(2)

[Peter]

In 4118:

document operations that invalidate references, views, iterators, etc. closes #974