Perform memory check on files before loading them in

[psavery]

This adds the initial logic for performing a memory check.

We will estimate the amount of memory loading a file will take (the estimation may be different for each file type), compare that to the available system memory, and present a warning to the user if we may end up using too much memory.

This still needs some work, and we definitely want to make sure it is cross-platform.

We may also want a way to disable this check, in case some users receive the warning often and don't want to see it.