AffineConstraints::merge(): allow mixed numbers

[peterrum]

No description provided.

[bangerth]

How do you deal with the situation where other_number is not convertible to number? Say std::complex to double?

[bangerth]

In other words, I'm curious about the use case. @tamiko introduced the template argument to the class because he wanted to deal with complex-valued problems. I just think that if you wanted to merge double and float constraints, why not just use the same data type right from the start?

[peterrum]

The use case is the same as in the case of

dealii/include/deal.II/lac/affine_constraints.h

Lines 605 to 607 in a1e45a1

	template <typename other_number>
	void
	copy_from(const AffineConstraints<other_number> &other);

Let's say we have an AffineConstraintd<double> and we want to initialize one AffineConstraintd<float>. In my current case, I have a MGConstrainedDoFs which always returns AffineConstraintd<double> but I would like to initialize a AffineConstraintd<float> for usage within multigrid.

[bangerth]

I did still have this question:

How do you deal with the situation where other_number is not convertible to number? Say std::complex to double?