Refactor GeneralMatrix and ConstGeneralMatrix classes

- Remove the GeneralMatrix(const ConstVector &) constructor, since it is hides
  a memory allocation (copying the ConstVector into a fresh Vector). This
  helped detecting and fixing several unneeded memory allocations. Some other
  memory allocations are now more visible (with an explicit Vector{}
  constructor).

- Add checks in GeneralMatrix(Vector, …) and ConstGeneralMatrix(ConstVector, …)
  constructors for verifying that the {Const,}Vector has unit-stride (this was
  an implicit assumption so far) and is large enough for storing rows*cols
  elements.

- Add GeneralMatrix::operator=(const ConstGeneralMatrix &).

- Delete ConstGeneralMatrix::operator=().