Fix generation of parameter file for selecting the output

Hi,

I am just moving around a bit of code (and clarifying the help).

Currently we cannot use swift like this ./swift -o test.yml to generate a parameters file with all the options related to the output selection. The code fails because it did not received a parameter file.

Therefore I moved the piece of code that generates the output file before the check for the arguments.

doc/RTD/source/CommandLineOptions/index.rst

@@ -52,8 +52,8 @@ can be found by typing ``./swift -h``::
     -n, --steps=<int>                 Execute a fixed number of time steps.
                                       When unset use the time_end parameter
                                       to stop.
-    -o, --output-params=<str>         Generate a default output parameter
-                                      file.
+    -o, --output-params=<str>         Generate a parameter file with the options
+                                      for selecting the output fields.
     -P, --param=<str>                 Set parameter value, overiding the value
                                       read from the parameter file. Can be used
                                       more than once {sec:par:value}.