sits_run_examples: Informs if sits examples should run
Description
This function informs if sits examples should run.
To run the examples, set "SITS_RUN_EXAMPLES" to "YES" using
Sys.setenv("SITS_RUN_EXAMPLES" = "YES")
To come back to the default behaviour, please set
Sys.setenv("SITS_RUN_EXAMPLES" = "NO")