Collection of helpful is_indepndent functions

[ChrisBeaumont]

If there are specific pruning functions that would be generically useful to use for Dendrogram.compute(is_independent), we should provide them. Some ideas:

• min_peak -- peak pixel passes a threshold
• min_sum -- sum of values passes a threshold
• contains_seeds -- contains at least one of a list of seed locations

It would be cool if these were composable, so that I could write something like

Dendrogram.compute(x, is_independent = min_peak(3) & min_sum(10))

And that would build an object that runs both tests. I'm not sure if &, and, or + makes the most sense to use.

[astrofrog]

Good idea. Regarding how to combine them, we could simply require a tuple of functions. This has the advantage that it will work for plain user functions too (which otherwise wouldn't understand the combination operators). So something like:

Dendrogram.compute(x, is_independent = (min_peak(3), min_sum(10)))

Although this doesn't allow different (and/or) logic. What do you think? Feel free to take a stab at implementing this, I won't be working on the code anymore for the next few days since I'll be travelling.

[ChrisBeaumont]

Closed via #78