AMPLExternalFunction: search for the specified library

[jsiirola]

Fixes #53

Summary/Motivation:

We previously performed no validation around the external function library location. This leverages pyomo.common.fileutils.find_library to search the filesystem for the specified library (looking in the CWD, plus directories specified the PYOMO_CONFIG_DIR, LD_LIBRARY_PATH, PATH).

Changes proposed in this PR:

• Search for the external function library, and if found provide the absolute path to to the NL writer
  -Add test for warning if the library is not found.

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[michaelbynum]

Looks good.

[mrmundt]

This has passed at least one test in each OS, plus is passing at least one jobs on Jenkins, so I am going to merge this PR.