refactor: move tests and fix pytest

[Tieqiong]

@sbillinge please check, thanks

With periodictable installed and a libdiffpy compiled from the repository against objcryst:

[sbillinge]

I will merge this because it is a huge PR, but I am not sure we want all the infrastructure for helping tests pass when different packages are missing. The point of tests is to fail when things are not right, and when dependencies are not available, things are not right, unless we decide that this is desirable behavior. @Tieqiong @bobleesj, can we meet and make a plan for this? I will open an issue to revisit this.

S

[bobleesj]

@sbillinge Yes, I can meet early next week, Monday morning? @Tieqiong

[sbillinge]

I am traveling Monday and won't be available. Please pick a time on my calendar from Tuesday onwards

[Tieqiong]

@sbillinge @bobleesj How's tomorrow looking? I imagine this will be a quick one. I want to make sure there's enough time to release srreal before the 15th as well.

[bobleesj]

I will be flying tmr (Sun). Mon works

[sbillinge]

I can't do Monday. @Tieqiong you and I can talk today if you are available. When would be a good time

[Tieqiong]

@sbillinge Can we meet sometime in the afternoon, like after 3 pm?