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*      2.0.0 NEWS  	  *
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1. add three new functions akashi_test, op_corre_CodonW and op_corre_NCprime.
2. change the name of three function proportion_index, op_highly and optimal_codon_statistics
3. split one function optimal_codons into two functions: op_highly and optimal_codon_statistics
4. change corresponding files, including DESCRIPTION, NAMESPACE, NEWS, manuel files and vignettes

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*      0.99.4 NEWS  	  *
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1. update R version

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*      0.99.3 NEWS  	  *
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1. change VignetteEngine error

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*      0.99.2 NEWS  	  *
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1. add a function genomic_gc3 in the package
2. change vignette


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*      0.99.1 NEWS  	  *
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1. import unlist function from BiocGenerics package
2. change the name of optimal_mutation_index to optimal_index, remove the conditional
test for genomic GC content should be lower than 50%