Alias setup env vars to their corresponding ECVL names

[simleo]

Some of the env vars that influence the PyECVL build have the same name as ECVL ones (e.g., ECVL_WITH_DICOM), others have different names (e.g., ECVL_EDDL, vs ECVL_BUILD_EDDL). For the next release, we could add aliases to the ECVL values in the setup script.

A change in the docs could mislead someone that's trying to install an older version, so we should keep reference to the old values around (e.g., this value for version < x.y, that value for version >= x.y)