Add issue difficulty level

[nbirillo]

Reorganize architecture to support issue difficulty level. We will have the same level for subcategories, but several inspections can have another level. We need to support these cases.

# Code style issues
CODE_STYLE = 'CODE_STYLE' - Easy
LINE_LEN = 'LINE_LEN' - Easy
# Best practice issues
BEST_PRACTICES = 'BEST_PRACTICES' - Medium
FUNC_LEN = 'FUNC_LEN' - Easy
BOOL_EXPR_LEN = 'BOOL_EXPR_LEN' - Easy
CLASS_RESPONSE = 'CLASS_RESPONSE' - Hard
METHOD_NUMBER = 'METHOD_NUMBER' - Hard
# Error-prone issues
ERROR_PRONE = 'ERROR_PRONE' - Hard
# Code complexity issues
COMPLEXITY = 'COMPLEXITY' - Hard
CYCLOMATIC_COMPLEXITY = 'CYCLOMATIC_COMPLEXITY' - Hard
INHERITANCE_DEPTH = 'INHERITANCE_DEPTH' - Hard
CHILDREN_NUMBER = 'CHILDREN_NUMBER' - Hard
WEIGHTED_METHOD = 'WEIGHTED_METHOD' - Hard
COUPLING = 'COUPLING' - Hard
COHESION = 'COHESION' - Hard
MAINTAINABILITY = 'MAINTAINABILITY' - Hard
# Info issues
INFO = 'INFO' 
# Others
UNDEFINED = 'UNDEFINED' - Hard
ARCHITECTURE = 'ARCHITECTURE' - Hard