Fix more warnings

[JohannesKauffmann]

• Add missing override
• Use size_t where applicable
• Use correct forward declaration
• Removed unused private member
• Fixed some tabs/spaces as a drive-by
• Use ++i instead of i++ in for-loops

[kblaschke]

Don't invest too much effort in the equation parser classes, this whole thing is on the kill list and in for a full rewrite.