`pamm` doesn't through warning when no offset variable present in data set.

[adibender]

When data is created with as_ped, the offset variable is automatically included in the data set, so when pamm is called, all is good.

However, when the ped data is created manually or offset column was deleted somehow, offset will be set to NULL implicitly assuming an at risk time of 1 for all subjects and intervals, which in most cases will not make sense.

This could occur sometimes, so better to warn users if offset variable is not present (especially since in pamm calls, this is not specified so users might not be aware, that offset is needed.