Deprecated StandardExpressionProcessor in favour of StandardExpressions class
Made ConversionService available from the StandardExpressions class
Refactored StandardExpressions, removed methods copied from parser and executor
Extracted IStandardExpressionParser interface
Deprecated StandardExpressionExecutor in favour of Expression's own execution methods
Created interfaces for standard expressions and structures
Closes #217. Finished refactoring on the Standard Expression processing mechanism