Up to now two such applications exist, afit beeing a very complex code for non-linear fitting (it makes use of Generalized Matrix Inversion and Tikhonov regularization) and can find up to 9 parameters. It is an old (1990) code of which the author lost the code and has restored it from a printed listing: so actually it is not so easy to use, also for the author, but runs and there is some hope to make it more usable.
The other app is cbm: it allows to compute the daylength in three
situations, namely 1)giving one latitude (degrees) and one date
(year,month,day); or 2) one latitude and more days (tipically starting and
ending month) [e.g 44.833, 2014, 4, 9; northern latitude,
year, april, september]; or 3) more latitudes (tipically an integer range of
latitudes, e.g. 15, 65, and a step (degrees), e.g. 2),year, month, day.
cbm also allows to select a so-called daylength coefficient, p.
As above-stated, the APPs are stand-alone codes, but they can run within Bong by the command DOS name-of-executable.