Feature Request: Add compatibility interface for checkpoint v0.4.x

[ckarrasalithya]

I'm trying to build a Docker image for AzureML with R 4.0. However, it seems that AzureML's scripts try to use the checkpoint library and fail because they are installing checkpoint version 1.0, but are trying to invoke functions from version 0.4.x (getValidSnapshots, setSnapshot) and fail because the functions can't be found. (If I configure the AzureML Docker image to use R 3.6 instead, then checkpoint 0.4.x is installed instead)

I implemented a hack/workaround to force it to use checkpoint version 0.4.x, but it really is a fragile hack that might break on future AzureML versions.

I believe it is critical for a "time travel" library like checkpoint to have a stable interface, otherwise it defeats the purpose if it's not possible to travel between some times because the time travelling library's interface has changed.

Therefore, I suggest adding a compatibility layer that would translate v0.4.x method calls and return values into the equivalent calls for v1.

Note that I already reported the issue to the AzureML team and they suggested that I report the issue here.

Thanks