Add option for user to specify wing load distribution manually

Desired capability or behavior.

This issue tackles a couple of TODOs in flops-based detailed wing mass to give Aviary the full functionality of allowing the user to specify the wing pressure distribution. Aviary currently only lets users specify triangular, elliptical or rectangular distributions. To give Aviary the same capability we should:

1. Add new variables to cover NETAP, ETAP and PVAL for user to specify a pressure distribution.
2. Allow load distribution factor = 0 and adjust the wing bending calculations to use the specified distribution.
3. Adjust the if/elif/else block to allow fload_distribution_factor to be a float with non integer values.
   This is probably a low priority issue since I think for majority of conceptual design applications assuming an elliptical lift distribution is completely reasonable as it is likely airfoil sections and twist can be used to produce it.

Aviary currently only has 3 options available (1,2,3) and cannot handle partial options like FLOPS:

Adding this functionality will also tackle this TODO:

Is your feature request related to a problem? Please describe.

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Associated Bug Report

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