Fix connect param

Created by: jtrebosc

There is a bug when trying to read fit param files with connected parameters. It was introduced when adding the ability to store unit for Position in param files. This pull request test if a parameter is a connected parameter tuple. It applies AxMult conversion only if it is not a connected parameter. There is a potential bug not fixed here : if the connected parameter refers to another dataset that is in different unit than the current one. So when adding data tab to a fit Widget, one should apply changeAxMult accordingly to the added dataset. I put a comment # need to convert units to first tab unit here at the position I believe it should be done.