Document random_pairs
calc_dists_dotprods(subset) defaults to NULL

this is easier to handle in calling functions
export random_pairs
When n=NA random_pairs draws all pairs
tests for random_pairs
OUT.ATTRS are irrelevant for us

* so set KEEP.OUT.ATTRS=FALSE
refactor calc_dists_dot_prods to return data.frames

* a list with one data.frame for each pair of neurons
* also add tests to verify this behaviour
rename random_pairs to neuron_pairs
choose random pairs (not all) from non_matching set

* we will rarely want all pairs from the random set
* better to choose a random set of the same length as our matching set
fudgefac -> epsilon

* not necessarily more descriptive, but slightly more professional!
* help emphasises this is a pseudocount
docs: add cross-refs