separate topological features by dimension

[corybrunson]

It will often be useful or necessary to treat topological features (persistence barcodes) of each homological degree separately, e.g. to learn the scale sequences over which to vectorize or to apply different vectorizations. This could be facilitated in at least 3 ways:

1. Add a degree parameter to step_vpd_*(), which will filter the input PDs before vectorization.
2. Write a new step (step_phom_degree()?) to take a PD column as input and output separate columns for specified degrees.
3. Add a logical parameter (by_degree?) to step_phom_*() to control whether the step outputs a single column of PDs or a PD column for each degree.

I believe (3) is most natural; but either (1) or (2) will be needed in case some vectorizations receive the full PDs while others receive only features of a specified degree. So i plan to implement both (1) and (3), to avoid having to create a whole new step.

Feedback is welcome!