[WIP] allow a skip interval in --blocks

[sslivkoff]

Motivation

want to be able to collect some blocks over an interval but not all of them

Solution

allow syntax --blocks START_BLOCK:END_BLOCK:SKIP instead of just --blocks START_BLOCK:END_BLOCK. this will use every SKIP blocks. so --blocks 100:200:20 would be blocks 120, 140, 160, 180, 200

[WIP] also allow --blocks START_BLOCK:END_BLOCK/N to take N samples from the interval. so --blocks 100:200/5 would be blocks 120, 140, 160, 180, 200

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes

[sslivkoff]

alternative implementation in #81