Make a Sharkfin Function

A sharkfin function is built in steps:

1. take the product of two sigmoidal functions

   • the first one rises around day $D$ with rate parameter $uk$

   • the second one decays around day $D+L$ with rate $-dk$

2. the product is raised a power $pw$

3. the result is scaled so that the maximum is $mx$ For the default values, the function looks like a shark fin.