22: constructor
In SoilR: Models of Soil Organic Matter Decomposition

the method constructs an object from a function a timerange where it is valid and a format

`map`	a function of one argument (time)
`starttime`	the point in time from which map is a valid representation
`endtime`	the point in time until which map is a valid representation
`lag`	a scalar describing the time lag. Positive Values shift the argument of the interpolation function forward in time. (retard its effect)
`format`	a string that specifies the format used to represent the atmospheric fraction. Possible values are "Delta14C" which is the default or "afn" the Absolute Fraction Normal representation