Stylistic points

[charlottelynch]

Things to consider:

1. whether we use add and radd or create a new function definition (if so, what name?) in AminoAcidProperties.py
2. I have created an overall class 'sbmlcore' in init.py - is this of any potential use?
3. Include type labelling throughout code?
4. Is there too much replication in AminoAcidProperties.py? Should we create an overall function for setting up the Pandas dataframe which could then be used for each amino acid property?

[charlottelynch]

5. Consider whether we want to include the option of using different hydropathy scales