Fix CMake scripts for installation and SYCL backend

[pzehner]

This PR aims to fix two aspects of the CMake setup:

1. Installation of the library with cmake --install/make install: this PR uses standard CMake practices to make the project configured with CMAKE_INSTALL_PREFIX=/path/to/install findable by a project configured with CMAKE_PREFIX_PATH=/path/to/install; and
2. Installation of the library with a SYCL backend: this PR adds missing CMake commands for this backend.

[yasahi-hpc]

@pzehner Maybe we also need to add install_test/bin/install_sycl.sh and add install test in workflow. But these stuffs will eventually be integrated differently in your new CI, so it may not make much sense.

[cedricchevalier19]

So kokkosFFT is the official name ?

I have a few comments about the build scripts, but not from this pr: you should not pass Kokkos Cmake options except for building Kokkos. When using an external and already installed kokkos, you should check the configuration from the actual install.

[yasahi-hpc]

As for the name, I propose kokkos-fft for docs (including repo name) and KokkosFFT for the rest (source codes, CMake). This may be the common way in Kokkos related libraries.

The install procedure as well as install test for Intel would be updated in new PR for CI by @pzehner ?

[pzehner]

@yasahi-hpc yes, I didn't add an install_test/bin/install_sycl.sh script as this work is handled differently in #29.

About the naming convention, at least on CMake side, I followed the same approach of Kokkos, for which the repository name is kokkos (lower case), and the CMake name is Kokkos (Pascal case, or at least title case).

[yasahi-hpc]

@pzehner Sure. If you are fine, I will merge this.

[pzehner]

I'm OK for merging.