Rename episodes to clean up the TOC

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Closes #21

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To keep track of the placeholder episodes removed:

13-vector-when-data-dont-line-up-crs.md questions:

• “What do I do when vector data don’t line up?” objectives:
• “Plot vector objects with different CRSs in the same plot.” keypoints:
• “In order to plot two vector data sets together, they must be in the same CRS.”
• “Use the GeoDataFrame.to_crs() method to convert between CRSs.”

14-vector-raster-operations.md questions:

• “How can I crop raster objects to vector objects, and extract the summary of raster pixels?” objectives:
• “Crop a raster to the extent of a vector layer”
• “Extract values from a raster that correspond to a vector file overlay with rasterstats.”

16-ndvi-time-series-raster.md questions:

• “How can I calculate, extract, and export summarized raster pixel data?” objectives:
• “Extract summary pixel values from a raster.”
• “Save summary values to a .csv file.”
• “Plot summary pixel values using pandas.plot().”
• “Compare NDVI values between two different sites.”

18-time-series-raster.md questions:

• “How can I view and and plot data for different times of the year?” objectives:
• “Understand the format of a time series raster dataset.”
• “Work with time series rasters.”
• “Import a set of rasters stored in a single directory.”
• “Create a multi-paneled plot.”
• “Convert character data to datetime format.”