Allow setting LB and UB constraints on variables as profiles over time

[tomjanus]

Currently, lower and upper bounds on variables are uniform across all time-steps and set as constants.
Allow those LB and UB constraints follow profiles in time.
Fix the IF statement in set_variable_bounds.m used to enforce that the last value of tank head $h_t$ is no less than the initial value, starting from line: if strcmp(var_name,'ht')