Pass functions as symbols rather than strings

[Bisaloo]

The officially recommended practice to pass functions is to use symbols rather than strings that we convert to symbols:

scenarios/R/helper_functions.R

Line 278 in cfb9576

fun.aggregate = lapply(summary_functions, as.symbol)

Passing functions as strings makes it difficult to do static analysis on code and impacts linting, seamless use of tools like the futureverse, etc.

[pratikunterwegs]

Thanks - {scenarios} functionality is under discussion, and this feature may or may not be continued, so I'll make this change when the package design is a bit more certain.