Small fixes to prevent negative microbial populations from propagating further errors#1200

jacobcook1995 merged 4 commits intodevelopImperialCollegeLondon/virtual_ecosystem:developfrom

1198-add-sensible-handling-of-negative-mycorrhizal-populationsImperialCollegeLondon/virtual_ecosystem:1198-add-sensible-handling-of-negative-mycorrhizal-populationsCopy head branch name to clipboard