Reconsider default number of buckets

[neubig]

Currently the default number of buckets is 4:

ExplainaBoard/explainaboard/analysis/analyses.py

Line 117 in 38db958

num_buckets=dikt.get('num_buckets', 4),

But this is probably too few when we're doing discrete bucketing. It'd probably be better to have the default be 4 for continuous and more (maybe 10) for discrete bucketing.