Reorganize into packages

[charmoniumQ]

We should reorganize the codebase into packages, so we can hide more of the implementation. The packages should expose what the other packages are "allowed" to see in their __init__.py.

The packages:

• types: Has types that get used by everyone else. Only specifies the interface of the data.
• reader: Specifies how to read ILLIXR data. Exposes only one function, read_trials(Path) -> Trials, defined in __init__,py, which calls every other reader. Other readers should be their own module. ILLIXR-specific information is only allowed in here.
• analysis: Specifies how to get the products of analysis. Exposes only one function analyze_trials(Trials) -> None, defined in __init__,py, which calls every per-trial and inter-trial analysis. Those should be defined in their own module. Graphics-specific information (labels, sizes) are only allowed in here.
• util: Utilities, all are exported. Each util function should be defined in its own module, but groups are permitted.
• check.py and main.py will remain in illixr.analysis. They will import other packages.