* The. The parameters "start" and "length" defines a sub array of the data array
* that will be used for the the fit. If start+length must not exeed the total length
* of the Data array. "start" must be chosen as close as possible to t0.
* The. The parameters "start" and "length" defines a sub array of the data array
* that will be used for the the fit. If start+length must not exeed the total length
* of the Data array. "start" must be chosen as close as possible to t0.