refactor in vitro

Signed-off-by: Andy Shapiro <shapiromatron@gmail.com>
Merge branch 'dev' into invitro
fixed migrations for invitro to delete old models before creating new
Merge branch 'dev' into invitro
Merge branch 'dev' into invitro
change pk to id to allow sorting in admin
completed updated data-model for invitro data
create flat TSV and Excel files amenable for data-pivot
add invitro to option for data pivots
use `cls` instance for class methods instead of specific class-type
add abstract AssessmentRootedTagTree model, used to create node-tree with

assessment root
create IVEndpointCategory for invitro models and associate with endpoint
include effect description tags in export
add benchmark values to data-pivot exports
Merge branch 'invitro' into dev
require variance to be specified for continuous endpoint datasets
staff-members can send emails to project-managers
reformat code in models for readability; remove blank default for factors
moved epi-choice options to relevant models