Skip GPU tests if cupy and numba not installed

See issue #50

Makefile

@@ -21,6 +21,7 @@ checks:
 	cd tests; pytest comoving
 	cd tests; pytest non-comoving
 	cd tests; pytest general
+	cd tests; pytest cuda-gpu
 
 linting:
 	flake8 ./
@@ -29,9 +30,3 @@ linting:
 dev_checks:
 	make checks
 	make linting
-
-# Tests that can only run on some systems
-gpu_checks:
-	cd tests; python cuda-gpu/test_gpu_binary_tree.py
-	cd tests; python cuda-gpu/test_gpu_ray_projector.py
-	cd tests; python cuda-gpu/test_gpu_sph_projector.py

tests/cuda-gpu/test_gpu_binary_tree.py

@@ -1,3 +1,8 @@
+import pytest
+
+cupy = pytest.importorskip("cupy")
+numba = pytest.importorskip("numba")
+
 
 def test_gpu_binary_tree():
     import cupy as cp

tests/cuda-gpu/test_gpu_ray_projector.py

@@ -1,3 +1,9 @@
+import pytest
+
+cupy = pytest.importorskip("cupy")
+numba = pytest.importorskip("numba")
+
+
 def test_gpu_ray_projector(show=False):
     """
     We compare the CPU and GPU implementations of ray-tracing.

tests/cuda-gpu/test_gpu_sph_projector.py

@@ -1,3 +1,9 @@
+import pytest
+
+cupy = pytest.importorskip("cupy")
+numba = pytest.importorskip("numba")
+
+
 def test_gpu_sph_projector(show=False):
     """
     We compare the CPU and GPU implementations of SPH-projection.