Fill null values with 'unknown'

[egrace479]

Fills null values in data with "unknown" and adds two test files with null values (one for image retrieval and one without):

• graphs.py: mapping function filters out values in lat-lon column that contain the string 'unknown', i.e., if either latitude or longitude is unknown.
• query.py:
  • get_data: gets information about included columns (from dashboard.py), sets null values to 'unknown', returns only suggested/required columns.
  • get_filenames: filters out any entries with 'unknown' Image_filename or file_url. Will return number of matching entries that are missing these values if no other images exist. Next update may do this when there are matching that can't be displayed in addition to the returned image(s).
  • get_images: Now will note that entries exist, but no image path or filename is available.
• Adjusted query tests to account for the changes in get_data and get_filenames functions.

Fixes Issues #29 and #30

[egrace479]

get_filenames returns ValueError indicating there are no images matching the user's selections, further states "Unknown filename(s) or path(s)" if there are still matching entries.