cmf_extract_subkernels: To extract subkernels
In conmolfields: Continuous molecular fields

To extract subkernels

1	cmf_extract_subkernels(kernels, ind_rows, ind_cols, mfields)

`kernels`
`ind_rows`
`ind_cols`
`mfields`

##---- Should be DIRECTLY executable !! ----
##-- ==>  Define data, use random,
##--	or do  help(data=index)  for the standard data sets.

## The function is currently defined as
function (kernels, ind_rows, ind_cols, mfields) 
{
    alphas <- kernels$alphas
    nfields <- length(mfields)
    subkernels <- list()
    subkernels$alphas <- alphas
    for (f in 1:nfields) {
        field <- mfields[f]
        subkernels[[field]] <- list()
        for (ialpha in 1:length(alphas)) {
            subkernels[[field]][[ialpha]] <- kernels[[field]][[ialpha]][ind_rows, 
                ind_cols]
        }
    }
    subkernels
  }