Descriptions can include `" ..  "` and when these are read,
they're converted to `" .."` internally. We need to make sure these are
escaped correctly so that when the internal XML string is passed to
parsers (like the minidom parser), it is still valid.

These are needed by the regression test for issue 20.

We could come up with a regression test that does not require these, or
use pyNeuroML to get these on demand, but for the time being I'm
checking these in.

If the startup handlers are run before them, then even though the code
does initialise variables, it does it in the wrong order. For example,
for `v = initMembPot` that is a `StateAssignment` that should happen
`OnStart`, `initMembPot` needs to be processed first so that `v` can
then pick up its value.

Fixes #20