Update the minimum required CMake version to 3.5.1
The minimum required CMake version is 2.8. This is very old and has resulted in various CMake version-specific logic in the Nektar++ CMake files.
As work has been done to support building on Windows 10 and other build-related updates have been made, this has required adding additional cmake policy configuration settings to the CMake files.
These updates and version-specific code in the CMake files by updating the minimum required CMake version. Ubuntu Trusty (14.04) was the Linux version with oldest packaged version of CMake, this was still at 2.8.12. Now that Trusty is out of its 5-year Long Term Support period, the minimum supported version of Ubuntu is 16.04LTS which includes CMake 3.5.1.
Centos 6/7, released in 2011/2014 respectively, both have cmake3 packages in the epel-release repository that are more recent than 3.5. Debian 8, the oldest supported Debian version only has cmake 3.0.2 available as a package but this Debian release goes out of LTS at the end of June 2020. Since some of the CMake version-specific logic relates to versions greater or less than 3.1, it seems sensible to move to a minimum version >3.1.
To maintain support for Ubuntu Xenial (16.04), moving to a minimum required CMake version of 3.5.1 seems like a sensible change.