Air drag model and test

[nataliaero]

Hi Kartik,

I wrote the code for the simple drag acceleration model and 1 test. I removed the comments from the test so that the output of the test is clean.

Natalia

[coveralls]

Coverage increased (+1.0%) to 87.74% when pulling 442c001 on nataliaero:master into 9d40bf7 on kartikkumar:master.

[coveralls]

Coverage decreased (-86.73%) to 0.0% when pulling b2e9c32 on nataliaero:master into 9d40bf7 on kartikkumar:master.

[coveralls]

Coverage decreased (-86.73%) to 0.0% when pulling c7832f0 on nataliaero:master into 9d40bf7 on kartikkumar:master.

[coveralls]

Coverage increased (+1.23%) to 87.96% when pulling 6e1531f on nataliaero:master into 9d40bf7 on kartikkumar:master.