#80 dialog for (sub)set reduction before time consuming procedures

[Lina Humbeck]

If the dataset is large there is a warning e.g. before clustering in the heatmap view or in the cloud view that it will take long and ask whether the user wants to continue. It would be nice to to give a hint to the option of the context menu of the subset column to randomly create a subset out of a given subset or to directly integrate such a option e.g. buttons for continue any way, cancel, make a random subset and then proceed. Maybe it would also be interesting to be able to make a subset of a (sub)set with only every x.th molecule.