Do not allow "on demand" ipm calculation when dealing with longer (> 1) operation chains with ad-hoc subc. selection

[tomachalek]

In these cases, we can only reliably compute the IPM if the appropriate text types (a.k.a. an "ad hoc subcorpus") are selected using the text type checkboxes, and only if there are no additional filters involved. However, some additional operations may be involved without disturbing the result - e.g. random shuffle, sample. So this requires additional careful consideration to get it right.