Test partial correlations endpoint with non-existent primary traits

Test that the partial correlations endpoint responds with an appropriate
"not-found" message and the corresponding 404 status code in the case where a
request is made and the primary trait requested for does not exist in the
database.

Summary of the changes in each file:
* gn3/api/correlation.py: generalise the building of the response
* gn3/computations/partial_correlations.py: return with a "not-found" if the
  primary trait does not exist in the database
* gn3/db/partial_correlations.py: Fix a number of bugs that led to exceptions
  in the case that the primary trait did not exist
* pytest.ini: register a `slow` pytest marker
* tests/integration/test_partial_correlations.py: Add a new test to check for
  an appropriate 404 response in case of a primary trait that does not exist
  in the database.

1 files changed, 1 insertions, 0 deletions

diff --git a/pytest.ini b/pytest.ini
index 58eba11..ba87787 100644
--- a/pytest.ini
+++ b/pytest.ini
@@ -1,6 +1,7 @@
 [pytest]
 addopts = --strict-markers
 markers =
+	slow
 	unit_test
 	integration_test
 	performance_test
\ No newline at end of file