Prepare tests for backward compatibility

[skodapetr]

We should have tests that will ensure backward compatibility of p2rank.
This is important for further development as contributors may not know why are certain things in the code.

For example:

    static String correctResidueCode(String residueCode) {
        //MSE is only found as a molecular replacement for MET
        //'non-standard', genetically encoded
        if (residueCode=="MSE")
            residueCode = "MET"
        else if (residueCode=="MEN")  // N-METHYL ASPARAGINE
            residueCode = "ASP"

        return residueCode
    }

is this specific for certain dataset/protein, etc...

In the ideal scenario we should have also test for every non-standard/invalid data p2rank can consume and internally sanitize.