Improve the performance of peppy

[rafalstepien]

• merge two similar loops into one
• get rid of accessing instance attribute multiple times

closes #388 closes #387

[nleroy917]

Your tests might be failing due to the attmap dependency in requirements-dev.txt. I had this issue as well... Check here: https://github.com/pepkit/peppy/pull/383#issuecomment-1155622589