Added Lots of Documentation

Added some nice to haves
Improved and Unified General Code Layout

MathKit/MPVariable.m

@@ -8,12 +8,7 @@
 
 #import "MPVariable.h"
 
-#import "MPParsedExpression.h"
 
-#import "MPToken.h"
-
-#import "MPExpression.h"
-#import "MPEvaluationContext.h"
 
 @implementation MPVariable
 
@@ -22,11 +17,13 @@
     self = [super init];
     if (self) {
         NSAssert(variableName != nil, @"variableName must not be nil.");
+        NSAssert(variableName.length > 0, @"variableName must be at least one character long.");
         _variableName = variableName;
     }
     return self;
 }
 
+
 - (NSDecimalNumber *)doEvaluation:(NSError *__autoreleasing *)error
 {
     return [self valueForVariable:self.variableName