Misc. typos

+ some are doxy others are just comment code.  
+ some minor whitespace and grammatical tweaks. 
+ app/SCL/* typos have also been submitted upstream (https://github.com/stepcode/stepcode/pull/366). So it's ok to merge them in to master.

src/Mod/Import/App/SCL/Part21.py

@@ -37,7 +37,7 @@ import time
 INSTANCE_DEFINITION_RE = re.compile("#(\d+)[^\S\n]?=[^\S\n]?(.*?)\((.*)\)[^\S\n]?;[\\r]?$")
 
 def map_string_to_num(stri):
-    """ Take a string, check wether it is an integer, a float or not
+    """ Take a string, check whether it is an integer, a float or not
     """
     if ('.' in stri) or ('E' in stri): #it's definitely a float
         return REAL(stri)

src/Mod/Import/App/SCL/TypeChecker.py

@@ -46,7 +46,7 @@ def cast_python_object_to_aggregate(obj, aggregate):
     return aggregate
 
 def check_type(instance, expected_type):
-    """ This function checks wether an object is an instance of a given class
+    """ This function checks whether an object is an instance of a given class
     returns False or True
     """
     type_match = False #by default, will be set to True if any match