New filter clean up

There were some oversights in previous MRs related to migration of filters to the new interface (including those originally exists in the legacy code). They don't affect the correctness of filters but should be addressed. For example:

1. auto field = ... => const auto& field = ...
2. move around or inline previous item to prevent accidental lambda capture.
3. [&, this] => [&]
4. Use the Field overload of CastAndCall*Field.
5. restructure contour/worklet and density_estimate/worklet.
6. explicit keyword for constructors taking a single parameter.
7. remove default constructor where it is necessary/correct/safe to prevent un-inited data members.
8. remove unnecessary #include.
9. other issues reported by clang-tidy.

There are many other issues reported by clang-tidy at the Worklet level. Unless they are very obviously true positive, they are left untouched.