Option to specify parallel-strings or series-groups

[aabills]

Hi! I ran into #167 recently, and realized there's a pretty easy fix by adding a keyword argument to just add a busbar on every row instead of just the top row. I'm pretty sure in practice most packs use the "series-groups" configuration rather than the "parallel-strings" configuration which is currently implemented in liionpack

Fixes: #167

[codecov[bot]]

Codecov Report

Merging #221 (c8b80ba) into develop (2148e85) will increase coverage by 0.01%. The diff coverage is 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #221 +/- ## =========================================== + Coverage 94.76% 94.78% +0.01% =========================================== Files 11 11 Lines 1184 1188 +4 =========================================== + Hits 1122 1126 +4 Misses 62 62 ```