Switch default preprocessor to pcpp

[ZedThree]

As this is a python package, we can install it by default, and so don't need to rely on cpp being installed on the host system

Fixes #567

@bkmgit I tried your podman file locally with this change and it seems to work, but maybe you want to double check it too?

[bkmgit]

Will check. pcpp has a bundled and slightly modified version of ply. It seems that ply will no longer be updated on pypi.

[bkmgit]

Added GFortran to the build environment in Fedora, which enables all tests to run and pass - https://github.com/bkmgit/ford/tree/fedora-ci . It
would be helpful to have that information in the README file rather than hidden in the documentation. There may also be cases where people use flang or lfotran. As such would make pcpp an optional dependency.

GitHub

GitHub - bkmgit/ford at fedora-ci

Automatically generates FORtran Documentation from comments within the code. - GitHub - bkmgit/ford at fedora-ci

[ZedThree]

I'm not sure that the bundled ply is an issue. This PR also means we don't need to worry about any compilers being available in the build environment. Seeing as the docs may be built on an entirely different system, I think that's a good idea. If users want to switch to flang or lfortran for the preprocessor, that would have to be set manually anyway.