catch cases where units defined with multiple prefixes actually parsed when they should not have (i.e., '1 mmm' would be parsed as '1 mm' or '1 mcg' would be parsed as '1 cg').
add definitions for 'mc' as a prefix alias for 'micro', useful since u and mu can occasionally be confusing.
Fix search and replace error that botched enzyme activity units, also add plural for this.
Merge pull request #98 from olbrich/add-mcg

Add mcg