Enable merge of multiple sfmDatas

[servantftechnicolor]

See alicevision/AliceVision#1807

Previous sfmMerge was only accepting 2 sfmData.
Here we enable use of N>0 sfmData.

[codecov]

Codecov Report

Attention: Patch coverage is 45.45455% with 6 lines in your changes missing coverage. Please review.

Project coverage is 69.96%. Comparing base (1f42ad3) to head (b691308).
Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
meshroom/nodes/aliceVision/SfMMerge.py	45.45%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2654      +/-   ##
===========================================
+ Coverage    69.93%   69.96%   +0.02%     
===========================================
  Files          121      121              
  Lines         7088     7084       -4     
===========================================
- Hits          4957     4956       -1     
+ Misses        2131     2128       -3     

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