Coping with non-clustered cases in contacts dataframe

[isaac-florence]

Because cases in the linelist who do not appear in the contacts dataframe with from as NA do not have their node attributes displayed, just their ID, it is necessary to include them in the contacts. However, {igraph} in the get_clusters function turns NA to 'NA' and so falsely considers non-connected cases as one cluster.

Although this does not change default behaviour it does allow for users to have a linelist returned which does not assign clusters to non-clustered cases.

The minor fix was required due to an issue in copying an epicontacts object but has now been tested and works as expected