Subsets based on boolean property

[andrea-bistacchi]

In different situations, it could be useful to select just a subset of an entity. This can be easily implemented using a scalar property defined on nodes (or cells?) that takes values 1 (=True -> belonging to subset) or 0 (=False -> not belonging to subset).

This will be hugely useful in analysis (e.g. select a subset from a set of dip data or a point cloud, see #29) and modelling (e.g. modify just a subset of a surface).

Actually, no update is required in the data structure to implement this system, but only tools to select subsets based on a property, and tools to select a subset of points (or cells) and attribute a property. These can be interactive selection tools (e.g. a lazo tool to select nodes within a polygon) or query tools based on combinations of other properties.