add seqtk sample task

seqtk.wdl

+task sample {
+    File sequenceFile
+    File? outFilePath = "subsampledReads"
+    String? preCommand
+    Int? seed
+    Boolean? twoPassMode
+    Float? fraction
+    Int? number
+    Boolean? zip
+
+    command {
+    set -e -o pipefail
+    ${preCommand}
+    seqtk sample \
+    ${"-s " + seed} \
+    ${true="-2 " false="" twoPassMode} \
+    ${sequenceFile} \
+    ${number} ${fraction} \
+    ${true="| gzip" false="" zip} \
+    ${"> " + outFilePath}
+    }
+    output {
+        File subsampledReads=select_first([outFilePath])
+    }
+}
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