doc: conda: add note about csh/tcsh

On Ubuntu and its derivatives, the csh shell at
/bin/csh, which is used in the shebangs of the Icepack scripts,
is not compatible with conda.

Mention that in the documentation and explain how to install tcsh as an
alternative, and configure the system such that /bin/csh points to
/bin/tcsh.

doc/source/user_guide/ug_running.rst

@@ -359,6 +359,16 @@ On GNU/Linux:
   # Close and reopen your shell
   
 
+Note: on some Linux distributions (including Ubuntu and its derivatives), the csh shell that comes with the system is not compatible with conda.
+You will need to install the tcsh shell (which is backwards compatible with csh), and configure your system to use tcsh as csh:
+
+.. code-block:: bash
+
+  # Install tcsh
+  sudo apt-get install tcsh
+  # Configure your system to use tcsh as csh
+  sudo update-alternatives --set csh /bin/tcsh
+
 .. _init_shell:
 
 Initializing your shell for use with conda