Decide what tskit functionality we want to expose as R functions

[gregorgorjanc]

See

tskitr/R/src/tskitr_minimal.cpp

Line 51 in 88985e1

int table_collection_num_nodes_zero_check() {

and

tskitr/R/inst/STATE_and_AIMS.md

Line 309 in 88985e1

### 4) Call `tskit` C API in R code in an R session or another R package (TODO)