fixed issue 27, AttributeError when calculating fuel flow for A21N

[rmonstein]

Fixing issue #27

There are minimal changes, but it needs to be checked if I used the package in an unintended way.

[rmonstein]

Additionally, I corrected the confusing text of an exception. If I understood the code correctly, the exception refers to the fuel model, not the drag polar.

[rmonstein]

What is still a bit confusing to me in the fuel flow model is the way how it is computed. The fuel flow seems to be computed solely based on one coefficient and the thrust ratio (lines 20 to 21 and its use in at_trhust). Is there any reference for that formula and the coefficients?
The paper "OpenAP: An Open-Source Aircraft Performance Model for Air Transportation Studies and Simulations" has, as far as I can tell, describes a different method to calculate the fuel flow.