Dweather/conan build

[weatherhead99]

This patch is intended to start modernising the CMake build system, just a little bit. If there is acceptance of these type of things I'll go further in future because modernising CMake build systems is a weird hobby of mine.

The reasons are:

• use of absolute paths based on environment variables is pretty annoying (when e.g. wanting to build with docker)
• the current system utterly fails if your cfitsio and ccfits are not installed in specific locations (this is true e.g. for fedora and suse based distributions)
• it does not currently support "true" out of source builds (i.e. it always dumps libraries and binaries in the source dir anyway), and because of environment variable computation required running from the "./build" directory, again quite annoying to write a consistent docker /container file for
• there is unnecessary definition of manual compiler flags which is taken care of by CMake if using their features as documented

Nothing here should break any existing buyild instructions. I have tried out those instructions in an ubuntu 22.04 distrobox and it still appears to work. But please let me know if anything looks concerning. I would like to have this merged so I can work on a reasonably simple Docker build process. Currently t hat build process would be highly compillicated.

[scizen9]

This is a good idea, but we are in the process of a major refactor of cameera-interface. We need to plan how to integrate this with the refactor, so, for now, I recommend you keep this on your branch for your current needs. We'll definitely want these kind of updates for the refactor, but we are focusing on the code structure initially and haven't dealt with the build yet.