Add reference to NRELs data disclaimer (#73)

Author: AdamRJensen

docs/source/notebooks/reference_dataset.ipynb

@@ -8,7 +8,7 @@
     "\n",
     "This notebook documents the quality control and subsequent removal of erroneous data in the 1976 Barstow irradiance dataset, which has been adopted as the de-facto standard dataset for shading studies of two-axis tracking collectors.\n",
     "\n",
-    "The dataset contains 15-min measurements from 1976 of the irradiance components: global horizontal irradiance (GHI) and direct normal irradiance (DNI). The dataset is available from the National Renewable Energy Laboratory (NREL) and can be downloaded [here](https://www.nrel.gov/grid/solar-resource/assets/data/f06.txt). Apley (1979) was the first to use this dataset and it has since been used in numerous studies (e.g., Cumpston and Pye, 2014; Meller, 2010; Pons and Dugan, 1984; Osborn, 1980).\n",
+    "The dataset contains 15-minute measurements from 1976 of the irradiance components: global horizontal irradiance (GHI) and direct normal irradiance (DNI). The dataset is available from the National Renewable Energy Laboratory (NREL) and can be downloaded [here](https://www.nrel.gov/media/docs/libraries/grid/f06.txt?sfvrsn=fe1796ab_1). Note that by accessing these files, you agree to abide by [NREL's data disclaimer](https://www.nrel.gov/disclaimer). Apley (1979) was the first to use this dataset, which has since been used in numerous studies (e.g., Cumpston and Pye, 2014; Meller, 2010; Pons and Dugan, 1984; Osborn, 1980).\n",
     "\n",
     "However, the dataset contains many periods with erroneous and invalid data (e.g., periods of unfeasible high irradiance values, incorrect tracking, and irradiance at night). Particularly, the measurements of DNI were prone to errors, which is the main irradiance component utilized by two-axis tracking collectors. As noted by Meller, Y (2010), different studies report different annual shading values using the dataset, which is \"believed to be due to the different treatment of missing irradiance data.\" It is the aim of this notebook to provide a quality-controlled version of the 1976 Barstow dataset that can be used as reference in future studies.\n",
     "\n",