Implement saving/loading different formats in g2o_viewer

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 91.17647% with 3 lines in your changes missing coverage. Please review.

Project coverage is 65.26%. Comparing base (f449b3d) to head (a31323a).

Files	Patch %	Lines
g2o/core/abstract_graph.cpp	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## pymem #820 +/- ## ========================================== + Coverage 65.13% 65.26% +0.12% ========================================== Files 342 342 Lines 13273 13296 +23 Branches 1306 1308 +2 ========================================== + Hits 8646 8677 +31 + Misses 4247 4239 -8 Partials 380 380 ```

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