Add code for tracking the minimum daily beta factor (btran), ELM

[mpaiao]

The proposed pull request adds the minimum daily beta factor (BTRANMN), using the same code as CLM5. In this pull request, BTRANMN is just a diagnostic variable, though one that is more sensitive to water stress than the regular daily/monthly averages of BTRAN. In the future, this variable may be used for drought deciduous phenology in FATES, and this variable is a candidate for the environmental driver controlling the abscission and flushing times.