BISMG:SarahS/jules merge

Notes on merging AW glacier code to current JULES trunk
 * Install tkdiff


 * Create 3 directories in the puma repository in my work space to contain
 * 1. A copy of the trunk to which changes will be merged to
 * 2. Andy Wiltshire's code
 * 3. JULES version 3 from which Andy made his changes

Queries on merging
 * snowpack.F90 - Do I need the #if_defined statement at the top of the file?