Map - default variable for bubble markers in megastudy is super-slow - think about improving default variable configuration

[bobular]

Despite the work done in VEuPathDB/EdaDataService#320 for VEuPathDB/EdaNewIssues#690 it still seems to take a long time to calculate the all/all proportion for the megastudy's species variable.

The current choice of default variable is the "first compatible featured variable".

We need to think about either adding new variable annotations, or some other way to select default variables?

Perhaps we could add more featured variables but in bubble marker mode the client filters out variables with a lot of values. The problem there is that any such featured variables have to be "below" sample.species in the entity tree, or they'll be picked up by donut/bar markers as the new default. I think we really want to keep species default for those.

TBH there isn't an obvious choice for a default bubble marker mode. Especially variables with 100% coverage. There are some great choices for specialised questions, such as mortality rate, genotype prevalences etc.

Tricky...