Function describing two exponential decline functions with a break point between them.
HS.solution(t, parent_0, k1, k2, tb)
| tb | Break point. Before this time, exponential decline according to
 | 
|---|
Other parent solutions: 
DFOP.solution(),
FOMC.solution(),
IORE.solution(),
SFO.solution(),
SFORB.solution(),
logistic.solution()