Scale spikes by `dt`

- Fixes #510.
- PES is now invariant to choice of `dt`.
- BCM and Oja are theoretically invariant to choice of `dt`,
  but in practice the output varies somewhat since these rules are
  so sensitive to the individual neuron activities. The difference
  is thus less pronounced for rate neurons.
Better error message for unprobable LearningRules

The BCM and Oja rules currently have no probeable parameters. Trying
to probe something on them gave a very unhelpful error message
that made it sound like no LearningRules are probeable. Fixed this.
Exposed some internals of BCM and Oja for probes