Added spack package file

[G-Ragghianti]

This is the package file that will be used to install fiber within spack. It is good to have this in the same repo as the main code so that the spack package can be updated in sync with the code base. The package.py is base don Piotr's previous work, but after working on it for a while I realized that the fft library would be better represented as a multi-value variant instead of separate variants for each library. One reason this is better is that it allows us to easily iterate over the list of supported libraries to reduce redundant configs in the package definition.

[luszczek]

Is this needed for CI? This is too early in development to have a Spack package but if CI gets simplified then why not.

[G-Ragghianti]

Is this needed for CI? This is too early in development to have a Spack package but if CI gets simplified then why not.

It isn't needed for CI, but I don't see any reason to not store the code right now in the repo even if we aren't going to immediately submit it to upstream spack. By writing the spack package to work with the current cmake build, it helped me to understand how the cmake may be improved to make it work better with spack.