use ref if path supplied

workflow/main.nf

@@ -117,7 +117,10 @@ process get_reference {
 
 workflow {
     // Select reference genome
-    if (params.reference == "hg38") {
+    if (file(params.reference).exists()) {
+        ref = file(params.reference)
+    }
+    else if (params.reference == "hg38") {
         ref = file("/project/apps_database/cellranger/refdata-gex-GRCh38-2020-A")
     }
     else if (params.reference == "mm10") {