Improve the algorithm of computation of the exposed area.

Improve the algorithm of computation of the exposed area.
The algorithm OCCTools_ComputeExposedArea has been improved by switching to working with compounds of Faces instead of Solids in the Gluing mode.
In the Gluing mode there is no need to work with solids, as the arguments are expected to be touching only and should not have any common volume. Working with faces helps to avoid errors while constructing the solids.
In this mode the result of the operation will be just the compound of faces.