Upgrade third party boost version to support building on Windows 10
With recent updates for building on Windows 10 (see !1143 (merged)), Nektar++ can now be successfully built against a binary boost install, with MPI support if required, using MS Visual Studio 2015/2017/2019. However, building Boost as a third-party library as part of the Nektar build fails on Windows. This is because the third-party version of boost (1.57) is too old to support recent MSVC versions.
Upgrading to a more recent third-party version of Boost shouldn't cause issues for other platforms so it should be ok to make this change across all platforms rather than just for Windows.
At the same time, the minimum required version of Boost will be left unchanged so that platforms that have older versions of Boost (but more recent than the minimum version required by Nektar++ - 1.56) can still use their standard package versions.
Tests using Boost 1.73, the most recent version at the time of writing, result in a large number of build errors in Nektar++ (although Boost itself builds fine). Initial investigation suggests that this has something to do with the Nektar++ build only being configured to require C++11 while Boost 1.73 requires C++14.
As a result, Boost 1.71 has been selected as the new third-party version.