Compilation error when Scalar is Eigen::half and not float

Since `class Matrix` is templatized

````c++
template
class Matrix final {
......
private:
...........
// The zero_point, i.e. which Scalar value is to be interpreted as zero.
// When Scalar is floating-point, this must be 0.
Scalar zero_point_ = 0;
};
````
I could have something like: `Matrix myMatrix;`

but then at compilation I get `error: no viable conversion from 'int' to 'Eigen::half' Scalar zero_point_ = 0;` since the zero isn't templatized

integer 0 and float zero are interchangeable so the above code works; but it's not the generic case for templates. I believe an implementation like `Scalar zero_point_ = Scalar{0};` is more generic

Similar fixes to other classes and parts of the code?