trident should be usable without ion balance files

[brittonsmith]

Currently, the trident import will hang in the welcome message screen if there are no ion balance files present. This makes it much more difficult to include trident as a dependency in other packages, especially in the context of automated testing. In addition to this, there are use modes of the trident package that do not require ion balance tables at all. Given this, I think it makes sense to move the welcome prompt to the first attempt to load an ion balance file rather than just inside import trident.

[chummels]

It seems to me that the only use mode for trident without ion_balance is in creating a spectrum from on-disk ionization fields. Typically codes only include H and He species in their native datasets, so this basically limits Trident's use case to H I absorption line spectra prior to getting the ion_balance files. My concern in not reminding the user to get the ion_balance fields at install-time is that they'll install trident and maybe not get them, and then go to run trident down the road and have the code fail out at that point. Is this a major concern right now in getting trident to work in automated testing? I thought we had it working with travis as is?

[brittonsmith]

As Issue #56 states, there are already cases where ion fields are defined, so there are plenty of situations where tables wouldn't be needed. This issue is not related to running automated testing for trident, but testing for other codes that may depend on trident, like yt_astro_analysis. As it stands, trident can not be identified as a dependency to be just installed from pip. One has to also create a config file, download data, and move it to the right place, otherwise testing just hangs on the trident import. I am facing this issue right now.

Can we have import trident just issue a warning if no data is present and move the prompt to when the data is actually needed? I think this would accomplish the same thing.

[chummels]

OK, @brittonsmith and I discussed this offline and came up with two solutions. In the short-term, we can leave the prompt as it is in the code and just add documentation for people wanting to use the code (e.g. as an external dependency in code testing frameworks). But in the long-term, we will remove the hanging behavior in trident when it is imported for the first time and replace it with a message indicating that the user should run trident.setup() or something which will initiate the existing behavior for downloading the ion_balance files.