Fixed remaining .coverage file after running 'make clean'.

Signed-off-by: Daniel Scheffler <danschef@gfz-potsdam.de>

Makefile

@@ -46,6 +46,7 @@ clean-test: ## remove test and coverage artifacts
 	## don't call coverage erase here because make install calls make clean which calls make clean-test
 	## -> since make install should run without the test requirements we can't use coverage erase here
 	rm -fr .tox/
+	rm -f .coverage
 	rm -fr .coverage.*
 	rm -fr htmlcov/
 	rm -fr nosetests.html