Add code required by sum-of-squares circuits over images

[loreloc]

Add the following features to the library:

• Embedding layer: very similar to the categorical layer, but with possibly complex outputs
• Add operators: product, integration and conjugation of embedding layers
• Merge concepts of constant layers and log-partition layer into a single one. This is because integrating an embedding layer yield something that is not the output of a log-partition layer
• Add a way to specify complex parameters to the image_data circuit template