Cleanup the FieldVector<K,SIZE>

This MR cleans up the FieldVector class similar to the cleanup of just the specialization FieldVector<K,1> in !1387 (merged).

The following changes are implemented

• Unify the indentation and spaces inside brackets, around brackets, and around for, if statements.
• Add constexpr to all functions where possible
• Add noexcept keyword where possible (sometimes with a condition)
• Unify the formatting of documentation comments
• Simplify the constraints on the constructors, without changing its behavior
• Cleanup of includes (remove unnecessary includes, order lexicographically, write out full path)
• Rename the include-guard to common form

This MR also contains a change discussed and implemented in !1345 (merged), which should be merged before.