Draft: Cleanup implementation of grid-functions

Summary

This is an extraction of some code changes from !345 (merged) to improve the readability and consistency of the code in the directory gridfunctions/

Major stylistic changes

• Use class instead of typename in templates
• Use std::size_t instead of unprefixed size_t
• No spaces between function name and argument list
• Spaces around assignment = sign
• Prefer using over typedef
• If possible (and not automatically provided) write deduction guide for class templates

Not consistently enforced styles

• Use curly braces for class instantiation instead of round braces (universal initialization), see http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-list
• share_ptr that are stored in a class should be passed by value to the constructor and then moved into the instance variable, see https://stackoverflow.com/questions/12002480/passing-stdshared-ptr-to-constructors?noredirect=1&lq=1 and https://herbsutter.com/2013/06/05/gotw-91-solution-smart-pointer-parameters/

Other changes

• Implement global evaluation function for CompositeGridFunction.
• Move implementation detail closestFaceIndex into the class FaceNormalGridFunction where it is used.