Refine Python runtime environment

[georgemccabe]

There are some inconsistencies with the Python environment that is used to run Python Embedding in MET.

• If the X.Y version of the Python instance in the user's PATH matches the X.Y version of Python used to install MET, then the packages from the user's version are used instead of the installed version. This means that if a required packages like numpy is not available in the user's version, then MET Python Embedding will fail unless MET_PYTHON_EXE is set.
• Python Embedding scripts that read point obs need to import met_point_obs.py, which is not available in the python path. There is an existing MET issue (Enhance the MET point processing tools to read the Python 'point_data' variable instead of just 'met_point_data'. #2285) to support reading point_data directly into all MET tools that would resolve this issue.
• It appears that the Python executable in the user's path is what is used to actual call Python Embedding logic (w/o MET_PYTHON_EXE), even though it may use the Python packages from the installed version (if the X.Y versions don't match). It would be good to ensure that the actual installed version is actually used in this case.

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