feat: change ModelSpec to allow for simple array of input/output var names

[chrispcampbell]

Fixes #494

This loosens up the ModelSpec interface so that simple projects (that don't use plugin-config or plugin-check) can provide a simple array of input or output variable names instead of having to conjure up full InputSpec or OutputSpec instances. This has no impact on existing projects.

As part of this, I made a few small changes to the build package APIs (which should have no impact on existing projects or plugins):

• Added ResolvedModelSpec interface; we now pass a ResolvedModelSpec to the plugin callback functions instead of a plain ModelSpec.

• Changed the InputSpec interface to make the defaultValue, minValue, and maxValue properties optional.

• Changed the ModelSpec interface to make the datFiles property optional (this no longer needs to be specified for projects that don't use dat files).

Also, one small behavior change to the plugin-check package:

• The default bundle template will skip inputs that don't have the defaultValue, minValue, or maxValue properties set in the InputSpec and will warn the user. This shouldn't happen too much because most projects will provide these values (i.e., via plugin-config).