🐛 for solid-free systems, NaN everywhere in the calculation of the number of nodes etc.

[maxlevesque]

Example:

 nb of solid interfacial nodes            0
 nb of fluid nodes        29791
 The total charge is set ONLY on the solid nodes at the interface (           0 /           0 )
 Internal sites (at the interface) =            0 , charge per link =                       NaN
 External sites =        29791  charge per link =  -0.0000000000000000     
 Salt concentration    0.0000000000000000     
 Init density values :
 p_solid =                       NaN
 p_fluid =   0.0000000000000000     
 m_solid =                       NaN
 m_fluid =   0.0000000000000000