Fix #10
Fix #10

The use of str.isdigit was splitting the composition string (H4N4free-1) into ['H','4','N','4','free-','1'] resulting in the free-1 part being ignored as 'free-' is not a building block. I modified the code to an str.isalpha that would put the '-' character with the numerical character, resulting in the list ['H','4','N','4','free','-1']
Tarskin/MassyTools