Suggestion to add init_sigma option to the amd tool

[schaedeli]

For the amd start model it is important the it does not crash. I have a dataset where the start model terminates with an extremely high OFV, while a reasonable fit can be obtained with an increased initial estimate of sigma (e.g. 0.2 instead of the default 0.09).
It would be useful to have an option for setting the initial estimate of sigma, in addition to setting init for the structural parameters.