Enable building with fpm

[joewallwork]

Closes #77.

This PR adds fpm support. https://fpm.fortran-lang.org/

I am currently building with

fpm build --cxx-flag "-cpp -std=c++17 -I${Torch_DIR}/include -I${Torch_DIR}/include/torch/csrc/api/include -L${Torch_DIR}/lib" --verbose

I get the following error but I think this might go away soon if I recall correctly (@TomMelt @jatkinson1000)?

././src/ctorch.cpp: In function ‘const long long int* torch_tensor_get_sizes(torch_tensor_t)’:
././src/ctorch.cpp:317:25: error: invalid conversion from ‘const long int*’ to ‘const long long int*’ [-fpermissive]
  317 |   return t->sizes().data();
      |          ~~~~~~~~~~~~~~~^~
      |                         |
      |                         const long int*
[ 25%]                     ctorch.cpp  done.

././src/ctorch.cpp: In function ‘const long long int* torch_tensor_get_sizes(torch_tensor_t)’:
././src/ctorch.cpp:317:25: error: invalid conversion from ‘const long int*’ to ‘const long long int*’ [-fpermissive]
  317 |   return t->sizes().data();
      |          ~~~~~~~~~~~~~~~^~
      |                         |
      |                         const long int*
<ERROR> Compilation failed for object " src_ctorch.cpp.o "
<ERROR> stopping due to failed compilation
STOP 1

Checklist

• [x] Fix long issue
• [x] Update metadata in fpm.toml
• [ ] Avoid duplicated preprocessor macro settings?
• [ ] Update docs
• [x] Test fpm build in CI
• [ ] Add separate fpm CI workflow (or possibly use matrix)
• [ ] Try out fpm test
• [ ] Add linting for .toml files?