Add a convenience function for `ixmp.Scenario.solve()` functionality

[glatterf42]

Solving a Run with GAMS requires three steps, essentially:

1. Write the data of Run to a format that GAMS can use
2. Invoke the GAMS solve mechanism
3. Read the solution data back into Run

We currently keep this functionality separate, i.e. we have one function for each of these steps. This allows for easier testing and understanding what these individual steps do.
However, during today's MESSAGE meeting, we received the request to provide a convenience function combining all of these steps into one function - just like ixmp.Scenario.solve() does.
Otherwise, people are afraid the workflow of quickly solving a Run becomes too complicated.

Another way out would likely be translating more GAMS code in message_ix to linopy (or similar), thus eliminating the requirement to read and write GAMS files, but this is unlikely to happen soon enough.

[glatterf42]

For now, we have opted to put the functionality of reading and writing GAMS files into IXMP4Backend. This was done mainly to keep ixmp4 from having to handle too many tasks at once.
This decision has implications for #12, #13, and #15, though: without having some sort of writing/solving logic, we can't showcase the transport tutorial as it usually happens (i.e. with setting up the Scenario and solving it, then displaying the results). However, maybe that's not necessary: we could instead provide a tutorial that showcases the parts that ixmp4 can handle by itself: setting up a Run, adding some data to it, and retrieving it back with various filters.

Alternatively, we could implement some kind of I/O and solver logic as @danielhuppmann suggested: as part of a submodule of ixmp4, e.g. called ixmp4-solver. This could then be used to add the tutorials as usual.