minor problem but need to be fixed

[TiangeZhu]

Error occurs when some scores in MEI format can not be successfully transformed into XML format.

Error content:

1. Error while loading from xml:invalid literal for int() with base 10: 'X1'
   (because of float, int and string problems)
2. Error while loading from xml:Both @num and @Numbase attributes are required on tags.
3. Error while loading from xml:unsupported operand type(s) for +: 'float' and 'NoneType'

Current solution:

Workflow.py, in def analyze_patterns_in_opus:
if score.m2_score == None, return empty dictionaries.
It basically works as a "continue"
YET: if the last opus of the corpus happen to have this error, then the function will return empty dictionaries to def analyze_patterns, then no stats will be printed by the end of analysis of an corpus.

Future solution:
Fix in Music21 library. Or, find a better solution in workflow.py.