Default user installation path

[michael-petersen]

Porting from EXP-code/EXP-examples#8, it does probably make sense to add more specificity to the user/ directory name (i.e. where the user module libraries are installed). I don't think this is particularly dangerous, since 1) we are checking inside EXP for compatible libraries when loaded, and 2) most installations are probably not pointing to a global space (e.g. /usr/local/), but if it does go badly, it will really go badly.

I think the suggestion (exp_user) is probably fine, but maybe we prefer something else?