Simplify the the specification of allocation details for matrices.

[rasolca]

A tune parameter for the default AllocationLayout has been added too.

[rasolca]

cscs-ci run

[albestro]

The main comment I have is about the possible semantic confusion we might have between MatrixAllocation and AllocationType. I'm not referring to the naming of the classes, but more on the current names of the functions.

[msimberg]

Only minor nits, this looks nice overall.

[rasolca]

As it was spread on multiple conversation I answer here.

@albestro
Please note that matrix allocation specifications, allocation layout and layout are different.

The previous use of the name MatrixAllocation happened to not be a good choice.
Therefore any change in the code from MatrixAllocation -> AllocationLayout is simply due to the rename of the enum. This can create a bias when seeing the change
MatrixAllocation allocation() -> AllocationLayout allocation()
But nothing has changed beside the name of the enum.

MatrixAllocation is now used to collect all the useful specs which can modify the way the matrix buffers is allocated. However not all the options have an impact on how the matrix is used. How the leading dimension was picked and (in the future) if the matrix is allocated on demand don't have any effect on how the matrix is used.

Regarding the allocation member I decided to keep the previous name.
layout (which can work in the MatrixAllocation unit tests) is in the matrix context a even worse option. Pre-allocated matrices use a layout object for the setup. Therefore the user would expect an even more detailed object returned.
Feel free to suggest any other meaningful option.

[rasolca]

@msimberg @albestro, I modified the MatrixAllocation constructor with the new version that is needed for adding the third parameter later. Please have a look. 3c90c00

[rasolca]

@albestro @msimberg please have a quick look to e0ddcee

[rasolca]

cscs-ci run

[msimberg]

Renaming looks as discussed, thanks @rasolca!

[albestro]

very minor consideration

Suggested change

	inline std::string copy_to_lower(const std::string& str) {
	std::string s(str);
	inline std::string copy_to_lower(std::string s) {