Redesign aesthetics interface (possibly tidyeval)

[lazappi]

The current interface for defining aesthetics using metadata columns requires a string giving the name of the column (e.g. node_AES) and a string giving the name of a function to summarise samples in each cluster (e.g. node_AES_aggr). This approach has the following limitations:

• Can't use anonymous functions (e.g. node_colour_aggr = function(x) {...}) (doing this has other issues around naming)
• Only one column can be used for each aesthetic
• Can't set other arguments in functions
• Many function arguments, need (at least) two for each aesthetic

How could we address some of these issues?

• Use tidyeval
  • Use non-standard evaluation, e.g. names instead of strings
• Specify aesthetics using a list
  • aes = list(node_colour = list(func = mean, x = colA), ...)
• Directly pass ggplot aes

Possible issues to consider:

• Allowing static aesthetics (e.g. `node_colour = "red")
• Checking function arguments exist
• Summarise columns
• Naming

[lazappi]

Allow direct passing of summarised node statistics for aesthetics, possible via a node_metadata argument. See #31 for example.