Avoid dumping the data to the console on print after `load_rodent_data`

[gavinsimpson]

The very first thing that a new portalr user (conversant with R) is going to do after running

tbls <- load_rodent_data(portal_path)

is to run

tbls

and thus dump the entirety (up to R's print limits) of each data frame in tbls to the console.

Instead, if load_rodent_data() returned an object with an S3 class ("portal_data_list" for example), then portalr could provide a print.portal_data_list method that did something useful (list the names of the loaded data tables) and suggest how to access the individual data frames.

For example something like (but better, prettier, using crayon and clisymbols):

tbls2 <- tbls
class(tbls2) <- append(class(tbls2), "portal_data_list")

print.portal_data_list <- function(x, ...) {
  x_name <- deparse(substitute(x))
  tbls <- names(x)
  writeLines("\nPortal data: a list with the following data frames:\n")
  print(tbls, ...)
  access_msg <- paste0("'", x_name, "$", tbls[1], "'")
  writeLines(paste("\nAccess individual data sets with e.g.", access_msg))
  invisible(x)
}

which gives:

> print(tbls2)                                                                                     

Portal data: a list with the following data frames:

[1] "rodent_data"    "species_table"  "trapping_table" "newmoons_table" "plots_table"   

Access individual data sets with e.g. 'tbls2$rodent_data'

[ethanwhite]

Address Gavin Simpson's issues on portalr

[ethanwhite]

@gmyenni - I agree with @gavinsimpson that this would be a good change to improve this natural entry point into the package and the design change shouldn't cause any downstream issues.

[ethanwhite]

Address Gavin Simpson's issues on portalr