Configure CPack to allow easy creation of RPM packages

[rosswhitfield]

Changes:

• By configuring CPack I can now easily create a RPM file, tested on RHEL9. This will allow easy deployment to multiple computers.
• Added an option to not install pgplot files, so you can just used the one installed by a system package.
• Also fixed a bug where is wasn't picking up the help files when installed in /usr/share/.

Example of making the RPM

cmake . -DINSTALL_PGPLOT=OFF -B build
cd build
make
make package

produces the file diffuse-6.17.02-1.el9.x86_64.rpm