Add info about segmentation size restriction in ReadtheDoc

[sharoncy]

In theory, there is no upper size restriction for the segmentations in Nutil. However, in practise - if they are very large - the user must restrict the no. of cores used, or work on a computer with high memory, otherwise Nutil will crash. Beyond a certain point there is no advantage to working with bigger segmentations so recommendations should be given on upper limits. To do

• improve the info in the ReadtheDoc about this
• add better error messaging in Nutil

[sharoncy]

Some info added here: https://nutil.readthedocs.io/en/latest/QuantifierInput.html#preparing-the-segmentations