Unit tests for operator overloading; fix scalar bug

[jwallwork23]

Closes #221.
Closes #224.

This PR implements unit tests for the overloaded operators acting on tensors using funit.

While implementing these tests, I discovered that pre/post multiplication by scalar, post-division by scalar, and scalar exponent don't work if the scalar is non-integer. This was an oversight by me, which is fixed in this PR. It turns out you can multiply/divide any tensor by a [1] tensor and torch treats it as a scalar, so that's what I do here.

In the case of exponentiation, it doesn't seem to be valid to have a tensor exponent. I revised the implementation to use a torch_scalar_t type and restricted attention to the integer case for now.

[jwallwork23]

Following our discussion in the meeting, I've now extended the power functionality to account for both integers and floats, replacing torch_scalar_t with torch_int_t and torch_float_t. I covered both cases with the square unit test and also added a square root one.

[jwallwork23]

[Rebased on top of main to pick up the restructuring changes]

[jatkinson1000]

Hi @jwallwork23 This looks great.
Am I correct that you've restructured the C++ to match the re-organising you did in the Fortran?

One observation (that is not to be resolved here) is that our online API docs are no longer very useful for basic users and are instead geared at developers.
In reality, all basic users want is the Interface docs, and the list they get above is somewhat daunting. There is an option to lost interfaces here but I don't feel it helps. I'm not sure if there is an easy solution to this and should perhaps create a new issue.

[jwallwork23]

Am I correct that you've restructured the C++ to match the re-organising you did in the Fortran?

Oh, I didn't do that actually. Can do in a follow-up PR, though.

[jwallwork23]

One observation (that is not to be resolved here) is that our online API docs are no longer very useful for basic users and are instead geared at developers. In reality, all basic users want is the Interface docs, and the list they get above is somewhat daunting. There is an option to lost interfaces here but I don't feel it helps. I'm not sure if there is an easy solution to this and should perhaps create a new issue.

That's a good point. I guess this is a downside of the fypp approach?

[jwallwork23]

One observation (that is not to be resolved here) is that our online API docs are no longer very useful for basic users and are instead geared at developers. In reality, all basic users want is the Interface docs, and the list they get above is somewhat daunting. There is an option to lost interfaces here but I don't feel it helps. I'm not sure if there is an easy solution to this and should perhaps create a new issue.

Opened #232.