Make it possible to use a custom metric on grid neighbors searches

[Tortar]

The idea is simple enough: allow the user to define a custom metric instead of the usual metrics (manhattan, ...), this could prove useful since the filtering would be "preprocessed" inside Agents.jl through offsets_within_radius and this would be more performant than doing a filtering after using a chebyshev metric (e.g. assume that a person wants to do a 2d search in a 3d space).

As a first step we could allow the possibility to choose a metric at the calling function level, instead of at the model level as it is currently done, even improving the flexibility of the library.