LoadSGM150:

 Load the SGM150 Lunar gravity model. 
 The model is normalized and the outputs unnormalized.
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   Form:
   gravityModel = LoadSGM150( fileName )
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   Inputs
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   fileName        (1,:) Filename

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   Outputs
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   gravityModel    (1,1) Data structure
                         .name (1,:) Model name
                         .mu   (1,1) Gravitational constant (km^3/sec^2)
                         .a    (1,1) Model earth radius (km)
                         .c    (n,n) Cosine coefficients
                         .s    (n,n) Sine coefficients

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