Minor fix to python API and gitignore

[saurabh1002]

Thanks for you amazing contribution @simone-ferrari and team. Firstly, the package builds without issues, which is amazing. I am currently running your pipeline to see how it performs, and I ran into some small issues which I propose to fix in this PR.

1. The line */build/* in .gitignore does not actually ignore the build directory.
2. The poses saved in KITTI format had a trailing whitespace at end of each line which was throwing an error (see image below) when passed to the commonly used evo pipeline for computing odometry evaluation metrics.

I propose a fix two the above issues through the commits in this PR.

[saurabh1002]

This change is to open the file in append mode.
The truncate call basically clears existing contents of the file if it existed before, which the write mode does by default, but not the append mode.

[simone-ferrari]

Thank you @saurabh1002 !