split redev Server and Client classes

[jacobmerson]

In the coupler I found that splitting up the server and client into separate classes had some nice advantages like a more obvious interface where functions are only available as appropriate on the server or client side. Another benefit with the coupler is that on the client side we don't need to link against all of the libraries that are needed for the internal data structures (Omega_h).

I think it would be worth thinking about if we should do the same type of split in Redev. This could be accomplished either by two completely separate classes, or via a template (use the process type enum as a NTTP).

Here's an outline of what that could look like:

template <ProcessType PT>
class Redev {
    // most methods we can just change the if statements to "if constexpr"
    void SomeMethod() {
        if constexpr(PT == ProcessType::Server) {
            // do server calcs
        }
        else {
            // do client calcs
        }
    }
    // only a few methods are only on client or only on server, so we can just SFINAE them out
    // if we have a lot of functions that are only on server or client we can completely split the client server class definitions
    
    // use dummy template argument since SFINAE only works in deduced context
    template <ProcessType Dummy=PT>
    std::enable_if_t<Dummy==ProcessType::Server,void> ServerOnlyMethod() {
        // do computation that only makes sense on server
    }
};

[jacobmerson]

Cameron and I had a good discussion about this and decided that it is worthwhile to split up the redev class. We arrived at the following solution:

1. template Redev on NTTP (ProcessType)
2. Use if constexpr so we can maintain send/receive code next to each other for easier readability
3. Use static_assert to disable functions that should only be available on the server or client side

static_assert seems to give the best error messages and also has the least complicated syntax .

Here is an outline of how that code looks:

template <ProcessType PT>
class Redev {
    // most methods we can just change the if statements to "if constexpr"
    void SomeMethod() {
        if constexpr(PT == ProcessType::Server) {
            // do server calcs
        }
        else {
            // do client calcs
        }
    }
    // only a few methods are only on client or only on server. Use static_assert to fail if used in the wrong context
    void ServerOnlyMethod() {
        static_assert(PT==ProcessType::Server, "This function is only available from  Redev<ProcessType::Server>");
        // do computation that only makes sense on server
    }
};