Added fasta_sampler.nf file

fasta_sampler.nf

+process sample_fasta {
+	input:
+file fasta from fasta_file
+	
+	output:
+file "sample.fasta" into fasta_sample
+
+	script:
+"""
+head ${fasta} > sample.fasta
+"""
+}
\ No newline at end of file