added a separate running.rst with instructions to run niche
removed the running instructions from installation.rst
added 'running' to the menu structure and put 'tutorials' before 'code tables' ('tutorials' is often used by new users, 'code tables' is more technical)
Requested by unfrequent users who could not find the instructions to run NICHE after the initial installation (was a bit hidden in installation.rst)
Requested by unfrequent users who could not find the instructions to run NICHE after the initial installation (was a bit hidden in installation.rst)