Add python equivalent for SFMExample_bal.cpp

[johnwlambert]

Adds Bundle Adjustment functionality to the Python wrapper.

Should provide the equivalent of the C++ example (https://github.com/borglab/gtsam/blob/develop/examples/SFMExample_bal.cpp)

Present ambiguity @dellaert -- is

typedef gtsam::PinholeCamera<gtsam::Cal3Bundler> PinholeCameraCal3Bundler;

or

typedef gtsam::PinholeCamera<gtsam::Cal3Bundler> SfmCamera;

preferred? We use both throughout codebase currently.

Introduces:

• PriorFactor for SfmCamera
• GeneralSFMFactor<SfmCamera,Point3>
  See Line 34 and Line 68 of https://github.com/borglab/gtsam/blob/develop/examples/SFMExample_bal.cpp

A later PR should include

• SfmData -> set up of collection is considered part of “initialization”. Can add .push_back() method to this struct, Unit tests of newly wrapped functionality

[dellaert]

Nice, but please don’t do the SfMCamera name change

[dellaert]

Please format entire file with google style

[johnwlambert]

I'm following black here for the multi-line imports. Is there a particular PEP number you could point me to for Google style?

[johnwlambert]

https://github.com/psf/black

[johnwlambert]

@dellaert Hi Frank, should be ready for a re-review now. Thanks!

[johnwlambert]

Hi @dellaert @akshay-krishnan, can we go ahead and merge this today?

@xwu4lab @swarrier246, it might be helpful for you all to see the factors we use here as well.

[swarrier246]

would it be helpful to mention what the input file contents and format will be?

[akshay-krishnan]

I think it would be nice to rename mydata to something more descriptive of the type of data.

[johnwlambert]

how about scene_data @akshay-krishnan ?

[johnwlambert]

@swarrier246 thanks for the review, just added an extended comment about what the BAL contents look like