fix some problem

[xiesc]

Awesome project! I test the code on both x86 and arm(Xavier), when compiled on arm without neon, there is a bug that a variable named L should be level.
I also found that when formulating information matrix for velocity prior, the bottomRightCorner is miswritten by bottomLeftCorner.
And there are some information matrix is indeterminate, which will cause ComputationInfo::NumericalIssue in Eigen::LLT<information_t>. As result, I found that there are some times square_root_information_ is not computed as expectation. Adding a tiny value in information matrix can relax this problem, however, a custom LocalParameterization for this situation may be more appropriate.
pls check if these modifications is right.