Add `describe()` method to `Population`, `Projection` and other PyNN classes

see http://docs.python.org/lib/node40.html

Partially done in r414.

After experimenting with various different templating tools, I decided just to go for the simple templating in the built-in string module. The complexity of supporting multiple, or arbitrary, templating tools seemed to outweigh any benefits (the main limitations of string templates are lack of logic within the templates and inability to descend an object hierarchy (e.g. $a.b.c)).

For reference, here is an example of using a more complex describe() implementation which has a fill attribute (the method returns fill(template, context):

import sys
import Cheetah.Template
import django.template
import django.conf
django.conf.settings.configure()
import cherrytemplate

sim_name = sys.argv[1]

exec('from pyNN.%s import *' % sim_name)

setup()

p1 = Population((5,5), IF_curr_exp)

prj = Projection(p1, p1, AllToAllConnector())

simple_template = """
==== Population %(label)s ====
  Dimensions: %(dim)s
  Cell type: %(celltype)s
  ID range: %(first_id)d-%(last_id)d
  First cell on this node:
    ID: %(local_first_id)d
    Parameters: %(cell_parameters)s
"""

cheetah_template = """
==== Population $label ====
  Dimensions: $dim
  Cell type: $celltype
  ID range: $first_id-$last_id
  First cell on this node:
    ID: $local_first_id
    #for $name,$value in $cell_parameters.items()
    $name: $value
    #end for
"""

django_template = """
==== Population {{label}} ====
  Dimensions: {{dim}}
  Cell type: {{celltype}}
  ID range: {{first_id}}-{{last_id}}
  First cell on this node:
    ID: {{local_first_id}}
    {% for name,value in cell_parameters.items %}{{name}}: {{value}}
    {% endfor %}
"""

cherry_template = """
==== Population <py-eval="label"> ====
  Dimensions: <py-eval="str(dim)">
  Cell type: <py-eval="celltype">
  ID range: <py-eval="str(first_id)">-<py-eval="str(last_id)">
  First cell on this node:
    ID: <py-eval="str(local_first_id)">
    <py-for="name,value in cell_parameters.items()"><py-eval="name">: <py-eval="str(value)">
    </py-for>
"""

print 'Simple'
print p1.describe(template=simple_template, fill=lambda t,c: t % c)
print 'Cheetah'
print p1.describe(template=cheetah_template, fill=lambda t,c: Cheetah.Template.Template(t, searchList=[c]))
print 'Django'
print p1.describe(template=django_template, fill=lambda t,c: django.template.Template(t).render(django.template.Context(c)))
print 'Cherry'
print p1.describe(template=cherry_template, fill=lambda t,c: cherrytemplate.renderTemplate(t, loc=c))

The implementation still needs to be fleshed out, i.e. extend the range of information that is returned by describe().