CR-10 - Rename DCN network metrics (#15)

In view of the forthcoming changes, please rename the existing network
metrics as:

dcn.degree
dcn.betweenness
dcn.closeness
dcn.clustcoeff
dcn.eigencentrality
dcn.pagerank
All sections of the code which use these metrics (such as calculating
correlation, descriptive statistics, regression, and factor analysis)
will now use these new names in files like dependent.csv,
independent.csv, and variables.csv. Old names (like degree, betweenness
etc.) should NOT be supported.