[core] parallelization: introduce the notion of node 'size'

* node size is an estimation of the number of element to process on a given node, that may be used for parallelization.
* replace the explicit reference to an attribute on a node (Parallelization.inputListParamName)
* size can be:
  * dynamic: depends on the size of the node connected to a specific Attribute
  * static

meshroom/nodes/aliceVision/ImageMatching.py

@@ -6,6 +6,7 @@ from meshroom.core import desc
 class ImageMatching(desc.CommandLineNode):
     internalFolder = '{cache}/{nodeType}/{uid0}/'
     commandLine = 'aliceVision_imageMatching {allParams}'
+    size = desc.DynamicNodeSize('input')
 
     inputs = [
         desc.File(