Global constants not available in all models

[dhensle]

Describe the bug
Global constants are not available in every model specification.

To Reproduce

1. put constant in constants.yaml
2. try to use that constant in the following models: location choice, destination choice, and tour scheduling. It will not be recognized

Expected behavior
We should always have access to our global constants

Additional context
Useful for things like applying a factor to convert, say, km to miles using a single constant throughout the modeling system.