Reducing rounding in `construct_mesh`

[Dieter Werthmüller]

construct_mesh does rounding for the skin_depth and min_width to meter precision. However, for high frequencies this yields to problem, as min_width can be below 0.5 meter, in which case it returns 0.0.

Three things TODO:

• Reduce rounding from meter to cm.
• Ensure min_width > 0.0
• Make precision a parameter in construct_mesh

Thanks @mariacarrizo for reporting!