Allow specifying a branch from where model_zoo data should be downloaded

Currently adding and testing models from the model_zoo is basically impossible, as the system only reads from the branch corresponding to the library version (if found), or the default master branch instead.

We should allow specifying a branch to use to download the data from an environment variable, so that unittests (also CI) can use it.

Name of the variable: SECML_ZOO_BRANCH

When this variable is set, it should override the branch for both cases, version branch and default branch, in load_model._dl_data_versioned function.

To parse the env variable use settings._parse_env.