Add the possibility to append data to a variable

[csala]

Sometimes it's needed to appended or merge the output of a primitive with its own input, or with some other variable.

Two possibilities exist for this:

• Add a way to indicate that the output needs to be appended to the input. This way, for example, a primitive can add columns to X instead of replacing it.
• Add a native primitive that allows the concatenation of multiple variables: X = pd.concat([X1, X2], axis=1)