Resume simulation from checkpoint

[rheiland]

Resume a simulation from a previous checkpoint:

• enhancement PR; only added code, didn't modify
• select a MultiCellDS file (output000*N.xml) as a starting point to continue a simulation
• does not handle intracellular model state
• does not currently recover "output*_graph.txt" data; rather, it relies on the next mechanics step to recreate that information
• if a model uses custom.cpp code that is not captured in the MultiCellDS data, it cannot be recovered
• community testing is desired

Refer to README in root/unit_tests/resume_sim

Big thank you to @rb-arroyo for being an early and ongoing beta tester and to @heberlr and @elmbeech for design discussions and insights.