Describe the computational grid layers as used in the 3Di Modeller Interface

[leendertvanwolfswinkel]

• Add a new file i_mi_computational_grid.rst, titled "Computational grid", which should be available under "3Di Modeller Interface", in between "Modelling" and "Simulating"
• Describe that the computational grid in the 3Di Modeller Interface is an export the most important data from the gridadmin.h5 file
• Refer to the ways in which it can be created; this has already been described elsewhere, just refer to the right pages.
• Mention that is is meant for viewing only, you cannot edit it directly.
• Header for each layer (Node, Cell, etc); short description of the layer + table of the names and description of the attributes (and, if applicable, the meaning of the attribute values)

Some of this information is available in the threedigrid documentation