Add PODVector::assign(value)

[ax3l]

Summary

vector.assign(vector.size(), 42);

is a bit verbose for a standard operation, even if it mirrors https://en.cppreference.com/w/cpp/container/vector/assign

Add another overload similar to setVal(ue) used in other AMReX containers.

Additional background

AMReX-Codes/pyamrex#222 (comment)

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate

[ax3l]

We can also avoid an overload and call this setVal(value) in symmetry with MultiFab, what do you think?

Suggested change

	void assign (const & T a_value)
	void setVal (const T& a_value)

[ax3l]

In MultiFabs, we further overload the assignment operator for amrex::Real and/or int, for convenience.

[WeiqunZhang]

I think assign is fine. It will still be asymmetric unless we add assign to MultiFab, which most versions would not make sense.