add the essential comments in project about ice-ba

[liuhaomin]

I am sorry that current readability is not good enough. We are working on it but it will take some time. What kind of comments do you think is most essential? I guess we will do that first.

[qzhai]

hi, Haomin, thanks for reply.
I think the comments of basic data structure, the class member and the core algorithm which includes local and global BA is very important to understand the code and algorithm, thanks all of you a lot.

[J2eremy]

Hi, how is this work coming along?
It's really difficult to understand the code without comments especially in local and global BA.
For example, the comments of variable naming rules in LocalBundleAdjustor.h will help a lot.
Thanks.