add g2o import export, and fixes, and Manhattan example

[dehann]

No description provided.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@62b5929). Click here to learn what that means.
The diff coverage is 64.28%.

@@            Coverage Diff            @@
##             master     #235   +/-   ##
=========================================
  Coverage          ?   23.37%           
=========================================
  Files             ?       36           
  Lines             ?     1660           
  Branches          ?        0           
=========================================
  Hits              ?      388           
  Misses            ?     1272           
  Partials          ?        0

Impacted Files	Coverage Δ
src/RobotUtils.jl	4.34% <0%> (ø)
src/CanonicalGraphs.jl	57.5% <85.71%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 62b5929...2fc6992. Read the comment docs.

[andreasnoack]

Codecov Report

Merging #235 into master will decrease coverage by 0.42%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #235      +/-   ##
==========================================
- Coverage   23.79%   23.37%   -0.43%     
==========================================
  Files          36       36              
  Lines        1639     1660      +21     
==========================================
- Hits          390      388       -2     
- Misses       1249     1272      +23     

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c535820...2fc6992. Read the comment docs.