Run a systematic test: RhoAir function in AirDC Library #95

Run a systematic test: RhoAir function AirDC Library
It is necessary to develop a test function that can test the correctness of the RhoAir implementation. The test function will compare the RhoAir output with the expected output for a set of input data. If the differences are within 1/1000, then RhoAir function passes the test.

To have a test function is helpful!