Kronecker Product addition to stdlib_linalg

[adenchfi]

Based on this issue, (#699)
I decided to go ahead and make a PR with this functionality. In case I go forward with the GSoC program, I figured the sooner the better.

I basically used the same format as linalg_outer_product for the .fypp source and for the unit tests. I confirmed I can build the stdlib with these additions with cmake, and that all unit tests pass.

The API and ordering convention used is similar to numpy.kron, though my name is slightly longer. Additionally, because there is both a Kronecker product and a Kronecker sum, I decided to use the full name kronecker_product.

It was suggested to go small with the PRs, and this is my first one, so I have not included kronecker_sum in this PR.

PS: There is no change to the README, I am not sure why there was anything to commit in that one. I was adding an SSH key to my machine's git and I suppose (?) the commands it had me do forced another commit.

[jvdp1]

Thank you for this PR. I only have minor comments regarding the code.
Also, could you edit the specs and add this implementation, please?

[jvdp1]

Specs look good. Thank you.

[adenchfi]

I believe there's nothing else for this particular PR?

[jvdp1]

LGTM. I have no additional comments.
hank you for this PR.

[adenchfi]

LGTM. I have no additional comments. Thank you for this PR.

Thanks for guiding me through the process! I will work on the Kronecker sum as well sometime.

[jvdp1]

Thank you @adenchfi
@14NGiestas Can you have a final look, please? Then I will merge it!

[14NGiestas]

@jvdp1 LGTM - I'm just wondering if it works with arbitrarily indexed matrices (as an edge case for tests).

[jvdp1]

@jvdp1 LGTM - I'm just wondering if it works with arbitrarily indexed matrices (as an edge case for tests).

I am not sure what you mean. Somehting like:

real :: mat(-5:7, 9:11)

[14NGiestas]

@jvdp1 LGTM - I'm just wondering if it works with arbitrarily indexed matrices (as an edge case for tests).

I am not sure what you mean. Somehting like:

real :: mat(-5:7, 9:11)

That was what I had in mind (just to be picky), but I tested and it works anyway:

program main
    real :: A(0:2) = [0, 1, 2] ! or A(-1:1) and so on
    call m(A)
contains
    subroutine m(A)
        real :: A(:)
        print*, lbound(A), ubound(A) ! 1 3
    end subroutine
end program

[jvdp1]

@jvdp1 LGTM - I'm just wondering if it works with arbitrarily indexed matrices (as an edge case for tests).

I am not sure what you mean. Somehting like:

real :: mat(-5:7, 9:11)

That was what I had in mind (just to be picky), but I tested and it works anyway:

program main
    real :: A(0:2) = [0, 1, 2] ! or A(-1:1) and so on
    call m(A)
contains
    subroutine m(A)
        real :: A(:)
        print*, lbound(A), ubound(A) ! 1 3
    end subroutine
end program

Thank you. I'll commit your last change and merge it.
Tahnk @adenchfi for your contributions