Add a check_install function

First of all, thanks a lot for this cool package!

I am developping a tool where I need to install programmatically `julia` and `pyjulia` in a `conda` env. This process is a bit complicated by the need to call `julia.install()` once `pyjulia` has been installed in `python`, as it seems that there is no tool to check if the installation has already been finished or not.
In this context, it would be very useful to have a utility tool in `pyjulia` that check if the install is completed or not and that does not depend on the install completion.
This could also allow for systematically calling `julia.install()` on import when the install has not been run before, removing one step to the install from the users (in which case such tool would just be a private functionality).

Do you think adding such function would be possible/interesting? Sorry if this has already been discussed, I could not find any reference to it.