Adjust amino acid and position based on Unimod.

[fcyu]

There are N-term and C-term besides amino acids. I add these two to the AminoAcid enumerate so that they won't be "NONE".

There are five kinds of positions based on Unimod help and the xml:

Position: Chosen from a controlled list of categories. Choose "Anywhere" if the modification applies to a residue independent of its position, (e.g. oxidation of methionine). Choose "Any N-term" or "Any C-term" if the modification applies to a residue only when it is at a peptide terminus, (e.g. conversion of methionine to homoserine). Choose "Protein N-term" or "Protein C-term" if the modification only applies to the original terminus of the intact protein, not new peptide termini created by digestion, (e.g. post-translational acetylation of the protein amino terminus). If Site was specified as "N-term" or "C-Term", then "Anywhere" becomes equivalent to "Any N-term" or "Any C-term". Required

Thus, I modified the Position enumerate.

[ypriverol]

@fcyu In the present implementation to handle the N-TErm and C-TERM of the peptide/sequence the user can do it by selecting the N-term position and NONE in the amino acid. However, if you prefer we can do this more explicitly by adding the amino acid N-Term and C-Term. Please be aware that we should check some functions such as

https://github.com/PRIDE-Utilities/pride-mod/blob/master/src/main/java/uk/ac/ebi/pride/utilities/pridemod/ModReader.java#L319

This function returns the modification across multiple representations UNIMOD or PSI-Mod. Please take a look to it and if it works well than we can accept the pull request.

@fcyu It would be nice if you can also include some unit tests related with this addition.