Use shared_ptr holder for Problem

[sarlinpe]

@B1ueber2y Without this fix, the following code results in a segfault for me:

ba = pycolmap.BundleAdjuster()
pb = ba.problem
del pb

The shared pointer returned by BundleAdjuster.problem is lost if pyceres.Problem is not declared to use the shared pointer holder.

[B1ueber2y]

Nice catch. Indeed, this should be fixed.