More reasonable default dtc in check.inp

[alko989]

At the moment ´check.inp´ defines ´dtc´ by the minimum diff of the data. This was done to be able to handle not-included missing values. But it is wrong in many cases, especially with mixed data.

Better alternative is to use the ´diff´s of ´timeC´ and warn the user that is automatically generated.

All places where assumptions about dtc are used should be checked