Add IsNativeDenom() function to DenomTrace

[colin-axner]

Summary

Add the function:

// IsNativeDenom returns true if the denomination is native, thus containing no trace history. 
func (dt DenomTrace) IsNativeDenom() bool {
    return dt.Path == "" 
}
        

Problem Definition

We need to check if the path is empty to determine if it is native. This is less readable then the proposed function.

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[crodriguezvega]

I will add it as part of #3019.