Add Support for Configuring BMI C Library for Formulation Without Explicit Path

[robertbartel]

As soon as #217 is finalized, it will be possible to configure formulations to be backed by some BMI-compliant model from an external C library. The configuration requires the location of the library file to be specified as part of the formulation config.

While configuring via an explicit library file path makes sense as an option, some alternatives should be added. Support for searching LD_LIBRARY_PATH or standard base directories base directories like /lib may be sensible.

Regardless, some more options that make this part of configuration easy should be added.

[robertbartel]

This relates to #334, though it probably still make sense to keep them as separate tasks.