pySim-shell: Add 'switch_channel' command

We've already had the 'open_channel' and 'close_channel' commands,
which were sent to (and acknowledged by) the card.  However,
those commands didn't affect the pySim-shell state, i.e. all
communication would still happen through the default channel '0'.

With this patch we introduce a 'switch_channel' command, using which
the user can determine which of the (previously opened) logical channels
shall be used by pySim-shell.

Change-Id: Ia76eb45c4925882ae6866e50b64d9610bd4d546d
Closes: OS#6230
diff --git a/docs/shell.rst b/docs/shell.rst
index dea0bb4..51fd49e 100644
--- a/docs/shell.rst
+++ b/docs/shell.rst
@@ -199,6 +199,12 @@
    :module: pySim-shell
    :func: Iso7816Commands.close_chan_parser
 
+switch_channel
+~~~~~~~~~~~~~~
+.. argparse::
+   :module: pySim-shell
+   :func: Iso7816Commands.switch_chan_parser
+
 
 TS 102 221 commands
 -------------------