Modified _parse_gdx_results in GAMS.py to replace _parse_special_value and Updated Import Statement

[AnhTran01]

Fixes #3624

Summary/Motivation:

When parsing the results of a model after it has been solved, the level and dual value are obtained through a series of if statements in _parse_special_values that may cause slowdowns. This PR added GAMS existing functions to handle data parser for these special values in _parse_gdx_results.

Changes proposed in this PR:

• Replaced _parse_special_values with GAMS special value parser in _parse_gdx_results.
• Updated attempt_import to have fallback to pre-GAMS-45.0 API if gams.core.gdx is not available.

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[jsiirola]

This adds a hard dependency on numpy. Importing numpy from dependencies will keep this as a "soft" dependency:

Suggested change

	from pyomo.common.dependencies import attempt_import
	import numpy as np
	from pyomo.common.dependencies import attempt_import, numpy as np

It will still make numpy a hard dependency if you are going to use GAMS, but that is OK (although GAMS's available() should probably check both numpy_available and gdxcc_available)