Respond to JOSS Review 2

[trchudley]

A number of minor updates in response to JOSS Review 2 (available at openjournals/joss-reviews#7149 (comment) and associated pdemtools issues details below).

Primary changes are:

• Minor corrections and updates to documentation/notebooks in response to Issues with supplementary data download  #17 Version number #18 How large an area is appropriate to work with? #19 Geoid clarification in the notebooks #20 Plots in flow_aware_hillshade notebook #23
• Changing the pdt.search() function to include the custom columns generated within the function in response to Include aoi_frac in results from pdt.search() #22
• Minor updates to the manuscript prose.
• A new version number (v.0.8.5) due to the updated search function. This will be pushed to pip/conda when the review process is complete.