Weird behaviour when hotstart variable not found

[CyprienBosserelle]

Issue

When specifying a hotstart file the model looks for specific variable zs or hh (That should be updated). If none of these are found, the model revert to a cold start but somehow this screws up the model zb!....

Solution to be implemented

Fix variable name mixup

Variable name to be explored by BG_Flood should be broader and the default z variable should be considered a zs.

Check coldstart

Defaulting to cold start if something fails with hotstart is good but whatever is happening now is not right.