Fixes for compiler warnings

[yvaind]

Fixed some warnings that would be triggered in downstream packages including header files from this repo when building with the '-Wall -Wextra -Wpedantic' compiler flags.

[ethzasl-jenkins]

Can one of the admins verify this patch?

[yvaind]

@goodfaiter I'm sorry, but you need to unapprove and review again. I added about twice as many new changes and we need to discuss a few things. Since these may be relevant for deciding what to do with other open-sourced packages, I also added you @remod as a reviewer.

• There are at least 3 different ways errors are logged/handled: std::cerr, std::runtime_error, and CHECK (a google macro defined in logging.h). I think we should adhere to one standard, the ideal one maybe being MELO. However, this would introduce another dependency.
• There are tons of virtual and overriden functions that are not implemented and will not even trigger a runtime error if used. There are a few places where an error is printed, but not in a consistent manner (see previous item). IMO all of them should print an error and we need to decide which way to go.
• The override specifier is not used a single time, although plenty of virtual functions get overriden.
• Adding clang tooling would be nice. But it also means an additional, new dependency.
• There are files that are not compiled when building curves, but only when building curves_ros. I fear that there may be even more files inside curves that only get compiled when used by another package.
• There are certainly more issues...

[yvaind]

I discussed with @remod and as you suggested @goodfaiter, we think it's best to address above issues (#81 , #80 , #79 , #78 ) in separate PRs.