Updating reference dictionary library/glossary

[brandynlucca]

Some variables and dataframe columns have strings such as *_num appended to the end of their names to signify the datatype or usage. A data dictionary/glossary/library already exists to some degree here:

https://github.com/uw-echospace/EchoPro/blob/f653ba74f445ad97c365bfc077bde602a3b8ab8b/docs/classes-datastructures.md?plain=1#L25

This should be updated to explicitly define the common formats/naming conventions (e.g. appended *_df, *_gdf, *_arr) to reduce any confusion concerning how certain variables should be accessed.

Originally posted by @brandynlucca in #176 (comment)

[brandynlucca]

Portions of this have begun to be incorporated into the ReadTheDocs documentation addressed in PRs #182, #194, #195, and partially #201.

[brandynlucca]

This was implemented in a previous PR and is included in the Core data structures documentation page. This issue can therefore be closed.