added process to fasta_sampler.nf

src/fasta_sampler.nf

 Channel
   .fromPath( "data/tiny_dataset/fasta/*.fasta" )
   .set { fasta_file }
+process sample_fasta {
+  input:
+file fasta from fasta_file
+
+  output:
+file "sample.fasta" into fasta_sample
+
+  script:
+"""
+head ${fasta} > sample.fasta
+"""
+}