Fix and document issues with template.cpp

Boost is a requirement for the template, so the FindGROMACS.cmake
needs machinery to deal with that. It will now find the Boost internal
to GROMACS.

pkg-config generally helps (if available and GMXRC is sourced), and
that use is now documented in the README.

Finding the GROMACS libraries and headers is also a requirement.  This
now works whether or not the user has sourced GMXRC or has pkg-config
installed. The user can over-ride this with CMAKE_PREFIX_PATH if they
want to.

The template needs to use the same compiler and compiler flags for
post-C++98 support, and this is now documented in the README. In
master branch, some of these issues are side-stepped, so take due care
when merging.

Fixes #1554

Change-Id: Id30cf5149ead4a3f719499e37776a00f08309afc