Define cache dir for xgboost models

[jonasscheid]

This feature request came up in https://github.com/nf-core/mhcquant/issues/317

Currently, the xgboost models are always downloaded to $HOME/.ms2pip. In some cases that could lead to permission issues, but the major advantage here would be in containerized settings, where containers are built from their image every time leading to downloading the xgboost models over and over again.

Would be great to have an option to specify a cache directory, where models can be reused.