update ctd information to also mention bgc (#268)

Author: j-atkins

docs/user-guide/assignments/Research_proposal_intro.ipynb

@@ -85,7 +85,7 @@
     "\n",
     "### CTD\n",
     "\n",
-    "A Conductivity, Temperature, and Depth (CTD) sensor is an instrument used to measure the physical properties of seawater in oceanographic and environmental research. It typically consists of sensors for conductivity (used to derive salinity), temperature, and pressure (used to derive depth).\n",
+    "A Conductivity, Temperature, and Depth (CTD) sensor is an instrument used to measure the physical (or biogeochemical) properties of seawater in oceanographic and environmental research. It typically consists of sensors for conductivity (used to derive salinity), temperature, and pressure (used to derive depth). CTDs can also be equipped with additional sensors to measure biogeochemical parameters such as dissolved oxygen, chlorophyll, and nutrients. \n",
     "\n",
     "Throughout a CTD deployment, the research vessels needs to stay in the same location. To calculate the time needed for a CTD deployment, take the following information into account:\n",
     "\n",