sanity check errors and warns #49
Description
- accum should error when less than 3 units
- optimize should warn when dominating code near grand mean
- construct should error when convos, units, or codes is empty
- construct should error when less than three codes
- accum should error when a code has no connections
- accum should warn when edges are empty
- tests should warn when groups have less than two units
- tests should warn when only one group
- ...