[r] add clustering + partialized function changes

[immanuelazn]

This represents a subset of changes created in pr #189 (and later, branch bp/pr189-review).

• Includes introduction of the partial function system, as arguments to other functions. Similar to what is used for parameterization for ggplot.
• Adds in cluster_cells_graph() clustering function, and changes the first parameter name of cluster_graph_*() from snn to mat to more accurately reflect the input type.
• Adds tests for clustering, and partial function system in clustering.