Validate all input files for all components before running anything?

[jpn--]

Currently, ActivitySim loads component inputs "on demand". For example, the settings file for trip mode choice is not read in until it is needed. If there is a problem with this file, it won't be discovered until it is used, which could be a long time after a model run is started.

Describe the solution you'd like
We should load and validate all input files at the beginning of a model run, so users can be alerted to problems promptly. This would require that users actually have correctly configured inputs for an entire model run before it is started, and could not, e.g. start a run knowing it will fail at component 9, but intending to collect and evaluate outputs that are generated before that. As a work around, if the user knows a fail will occur at component 9, they could just disable that component (and all later steps).