Function SelectSources 

pub unsafe extern "C" fn SelectSources(
    nodes: *mut *mut nodeT,
    mag: *mut *mut f32,
    ground: *mut nodeT,
    nflow: c_long,
    flows: *mut *mut c_short,
    ngroundarcs: c_long,
    nrow: c_long,
    ncol: c_long,
    params: *mut paramT,
    sourcelistptr: *mut *mut *mut nodeT,
    nconnectedarrptr: *mut *mut c_long,
) -> c_long

Create a list of node pointers to be sources for each set of connected pixels (not disconnected by masking). Return the number of sources (ie, the number of connected sets of pixels).