Add FV slope limiter methods to some projects
Implementations taken from the Hesthaven 2018 book.
This PR implements
- the MUSCL scheme for the ScalarEq
- a general slope limiter method based on SSPRK3 time stepping for the EulerEq; we include all the limiters given in sec 10 of the book.
The implementation is a bit dirty, in the sense that it does not utilize the @with_signature
interface.
So this will need refactoring at a later stage, but should wait till we know how to wrap general FV methods into one implementation.