CRAN version 0.2.6

Major changes:

• Added new dodgr_centrality function, and associated helper functions.
• Added new dodgr_flows_si function
• Added new parameter norm_sums to dodgr_flows_aggregate
• merge_directed_flows renamed to merge_directed_graph, with added option
  of specifying columns to merge.
• Added new pairwise parameter to dodgr_distances; see issue #127
• Added new function dodgr_insert_vertex to add new vertices to graph; see #40
• Removed "radix" heap option

Minor changes:

• switch off examples that caused previous CRAN failures
• fix bug in dodgr_dists when number of from points >> number of to points
• fix bug in weight_streetnet.sc that prevented keep_cols from working