Add global parameters

[ceball]

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)

[ceball]

Oops, just made a bunch of edits (now reverted) because I mixed up global/script param issues! Please ignore.

[jlstevens]

I've not been following closely but I want to make sure you are aware of this approach which we used to set parameters via environment variables for generating batch reports.

[ceball]

Thanks. Yes, I was using that myself in the past :)

The edits here are just that I mixed up issues while distracted with too many tabs open!