Incorrect use of rvalue references and std::forward

See: https://github.com/boostorg/test/blob/11a8d379b473ebe67dcc7792b6411ba312f60398/include/boost/test/data/monomorphic/grid.hpp#L96

Which boils down to

```cpp
template
class grid {
//! Constructor
grid( DataSet1&& ds1, DataSet2&& ds2 )
: m_ds1( std::forward( ds1 ) )
, m_ds2( std::forward( ds2 ) )
{}
};
```

Here `DataSet1` and `DataSet2`, while template parameters to the class, are not template parameters to the function.

As a result, this formulation of the constructor *always* takes these two parameters as rvalue-reference.

This can be corrected in 3 ways

1. Take both parameters by value
2. Make the constructor a template function
3. Make additional constructors that represent the cross-product of const& and && for each parameter. In this case, resulting in 4 functions.

I see that this same mistake is done in more than one place, it's not only grid.hpp where it happens.