add unit test for writeBambuOutput and update bambu output

[lingminhao]

Here I add some unit tests for the writeBambuOutput() function. It tests whether:

• the prefix is as expected
• the output files go to the correct output path
• the file extensions are correct
• the column names for the count outputs include "GENEID" and/or "TXNAME".
• the sample columns for count output files are numeric.

Additionally, since the theta column was removed from the Bambu output, some of the internal/external data would also need to be updated.