Add analytic solution for Laghos Sedov

[pearce8]

• Depends on implementation in Laghos Add analytical Sedov shock solution CEED/Laghos#204
• In Laghos, add the option to run till convergence to some tolerance
• In docs, add parameters to run till convergence https://software.llnl.gov/benchmarks/12_laghos/laghos.html#running
• Study performance of runs till convergence (how long on different systems? is it the same number of iterations?)
• In docs, add validation section https://software.llnl.gov/benchmarks/12_laghos/laghos.html#validation
• Check if FOM takes convergence into consideration, and the number of time steps is reported in the output and Caliper

FYI @kennyweiss.