- simplify_indexed() simplifies c*(M).i.j -> (M').i.j (c: numeric, M: matrix)

- simplify_indexed() simplifies c*(M).i.j -> (M').i.j (c: numeric, M: matrix)
  with M'=c*M
- new member function matrix::mul(const numeric &other) to multiply matrix
  with scalar
- lst_to_matrix() takes a "lst &" instead of an "ex &"
- added constructor of matrix from flat list
- added function diag_matrix() to construct matrix from list of diagonal
  elements