Pymem save level

[RainerKuemmerle]

Implements a solution to #954

[codecov]

Codecov Report

❌ Patch coverage is 84.21053% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.76%. Comparing base (22705c3) to head (ccbd0cf).

Files with missing lines	Patch %	Lines
g2o/core/io/io_g2o.cpp	45.45%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            pymem     #955      +/-   ##
==========================================
+ Coverage   72.72%   72.76%   +0.03%     
==========================================
  Files         346      346              
  Lines       13027    13064      +37     
  Branches     1238     1242       +4     
==========================================
+ Hits         9474     9506      +32     
- Misses       3120     3124       +4     
- Partials      433      434       +1     

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