Add global parameters

From Jim:

> We also [want] global parameters, with an object in the main namespace that
> collects options specified on the commandline. I.e., any commandline option should
> have a corresponding parameter, and changing that option should have a direct
> mapping to that parameter, so that such parameters can be changed in a .rc file,
> from within a script, etc.

From Jean-Luc:

> Converted from SourceForge issue 3537800, submitted by jlstevens
> Submit Date: 2012-06-25 14:49 GMT
>
> [...] global parameters that are relevant to the whole simulator (eg. OpenMP
> settings) would be very useful. [...] global_params should be implemented with the
> following properties:
> - At the commandline any option '-x=y' should set global parameter 'x' to value 'y'
> - Global parameters belong to the global namespace so that you can put the line 'x=y'
> in your topographica.rc file to always set global parameter 'x' as 'y' by default. Any
> behaviour set in the rc file is overridden if explicitly set on the commandline.
> - At the IPython prompt, entering 'x=y' should set global parameter 'x' to 'y' for the
> reminder of that session (ie. it should be possible to rebind global parameters at any
> time)
>
> If this were implemented it would be possible to specify options in the topographica.rc
> file instead of having to specify these options each time Topographica is invoke. All
> the current options should be implemented this way (ie. -i,-v,-d,-l,-g, --pdb, -c, -p, -a)
> with the exception of -h/--help.

(collected from #12 and #39)