New PROPOSAL installation process. Change from gitsubmodules to conan.

There are many good reasons why a package manager helps to install PROPOSAL. So I will try it. After the cubicinterpolation dependency is in the conan center repo, I will add PROPOSAL. Then in the normal case, there is no need to build PROPOSAL anymore. Conan will fetch the right binaries and only build it, if there is no perfect match.

References:

• cubic interpolation PR
• proposal conanfile to install

Also take the new interpolation methods into account.

This is related to Issue #315 (closed)