control_f: Flexible decline function transmission rate function
In a-henderson91/vectorbornefit: MCMC fit SEIR model of vetor borne transmission

Sigmoid function to allow drop in transmission rate

1 2	control_f(time, mask = 0, base = 1, grad = 0.1, mid = 0, mid2 = 1, width = 30)

`time`	t in time.vals of calc
`mask`	Strength of reduction in transmission rate. Defaults to 0 (i.e. no effect)
`base`	Height of starting point of function. Defaults to 1
`grad`	Strength of decline in transmission once begun. Defaults to 0.5
`mid`	Point at which decline can begin. Defaults to 0
`date0`	reference index value for start of transmission. Defaults to 0