Option to specify starting point

[bencebecsy]

This came up for specifying source parameters based on F_e or F_p statistic so we start close to a good solution. But it can also be useful for other things. One possibility to have this as a generic option would be that the user can specify any parameter with a dictionary, and whatever they don't specify will be initialized as usual. Because sometimes you may not want to specify everything, and would want the usual initialization to happen for some parameters.