Combine print() and print_v() into a single write() that takes what-to-print as an argument

[apdavison]

Some simulators allow writing variables other than spike times and membrane potential to file. To support these, we should have a single function and a single Population method for writing variables to file.

Also, python tends to use write(), rather than print(), for this kind of thing.

This should be done in a separate branch until we've made a release with the current API.

[apdavison]

Began doing the groundwork for this in the nest2 module by factoring out a lot of the record/print stuff into single methods/functions used for spikes, membrane potential, etc. Now need to do for the other modules.

[apdavison]

Postponed to 0.8, as there are already enough API changes in 0.7