Problems with CTDClasses failing

[celiosantosjr]

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As far as I could research, the problem is the same with the function CTDD previously: "extractCTDD fails when less than four amino acids maps to any one group". But seems as far as it was fixed in the CTDD and also others CTDC and CTDT, it was not fixed in the part of the algorithm that deals with classes coming from the user. Could you fix that please?

[nanxstats]

@celiosantosjr -yes, would you mind providing a minimally reproducible example so that it's easier to verify if the issue is fixed? Or even better -- please feel free to create a pull request for your patch since the fix seems pretty straightforward.