Set new default extension for gendata files

[tralsos]

In grav_subs_points the files that are exported to use as GEN_DATA files have a default extension "_1.txt" including both the report step number 1 and the .txt extension.

There has been a change in ert where the report step is no longer necessary when specifying GEN_DATA, hence the default extension can be set to ".txt", but still allow for defininf a different extension.