New features to simplify exports

List of planned features in this MR:

• Add a new FuseToMetaOps recipe, a one-liner match and fuse;
• Add data type to GenericOperator (actually just allow it);
• Add a removeIdentity() recipe (added also a generic removeNode() recipe);
• Add data format to Aidge.

Proposal for data format:

• The default data format is Default, meaning no data format was explicitely specified. It is assumed to be NCHW;
• The user can specify an explicit format for the tensor and the operator;
• It is up to the implementation to check if the input/output format are compatibles (there is currently no check);
• New Tensor::setDataFormat() and Tensor::copyTranspose() functions are provided;
• Implementation of Operator Transpose was moved in Tensor::copyTranspose() and now uses this function;
• New recipe explicitTranspose() to automatically insert Transpose operator where needed (it requires data format to be explicitely specified and will not insert anything before or after Default data format).

@vtemplier Can you check this list and maybe add other ideas?