Moving gsw to general import section

Also changing particle.depth to particle.z throughout, following #2303

Author: erikvansebille

docs/examples/tutorial_gsw_density.ipynb

@@ -31,6 +31,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
+    "import gsw  # The package to calculate density from temperature, salinity and depth\n",
     "import numpy as np\n",
     "import xarray as xr\n",
     "\n",
@@ -78,7 +79,7 @@
     "    pclass=GSWParticle,\n",
     "    lon=[32],\n",
     "    lat=[-31],\n",
-    "    depth=[200],\n",
+    "    z=[200],\n",
     ")"
    ]
   },
@@ -98,11 +99,9 @@
    "outputs": [],
    "source": [
     "def ParcelsGSW(particles, fieldset):\n",
-    "    import gsw\n",
-    "\n",
     "    particles.temp = fieldset.thetao[particles]\n",
     "    particles.salt = fieldset.so[particles]\n",
-    "    pressure = gsw.p_from_z(-particles.depth, particles.lat)\n",
+    "    pressure = gsw.p_from_z(-particles.z, particles.lat)\n",
     "    particles.density = gsw.density.rho(particles.salt, particles.temp, pressure)"
    ]
   },