Fixes #22. At least on my local machine.

The reason for segfaults in `general_product_impl` is this line:
```
    auto vtilde = lhs.my_matrix() * rhs;
```
`my_matrix()` member function returned a temporary which was destructed
after the assignment (see Eigen's docs page about the use of `auto`).
You were then trying to access an already destructed matrix in line 101
which, obviously, didn't go well.
There are numerous solutions to this issue, but the simplest one that as
a by-product should also increase the performance, is to let `my_matrix`
return a reference to the stored matrix rather than a copy.