Do not silently start downloading missing bits assuming installation in hard-coded default locations

I have jnml installed in the right location: /usr/bin/. However, because OMV has the default location hard-coded as ~/jnml/jNeuroMLJar/, if it doesn't find it there, it just starts to download the thing. There's really no reason why users should have individual directories in ~/ for each software. This is not a standard location (not FHS or XDG or any other standard that I'm aware of). Most people will have a specific folder where they install software locally. So, this needs to be improved:

We should:

• first check a list of sane default locations: such as $PATH on Linux systems to see if the necessary command is available
• if the command is not found, we should inform the user and ask them to explicitly set the necessary environment variable to point to their installation,
• a specific value of the environment variable could be used to request OMV to download the artefact itself. For example JNML_HOME="omv-download".

No file or test is named in the issue. Start by locating the OMV code that resolves jNeuroMLJar and handles JNML_HOME, then inspect how its current fallback download works. Done means standard locations are checked, missing installations produce clear guidance, and an explicit download value remains supported.