Adding support for execution on slurm grids

- Adding support for SlurmTaskExecutor in `daggyd` if DAGGY_ENABLE_SLURM is defined.
- Renaming some test cases
- Enabling compile-time slurm support
- Adding slurm documentation

tests/unit_dagrun_loggers.cpp

@@ -54,7 +54,7 @@ TEST_CASE("Filesystem Logger", "[filesystem_logger]") {
 }
 */
 
-TEST_CASE("ostream Logger", "[ostream_logger]") {
+TEST_CASE("ostream_logger", "[ostream_logger]") {
     //cleanup();
     std::stringstream ss;
     daggy::loggers::dag_run::OStreamLogger logger(ss);