Add loading of downloaded csv files

Downloaded csv files can now be loaded those back into the
data_retrieval tool
Prevent long load times to download large datasets multiple times
Major updates to using local files

Add support for HiSPARC Public Database source files
Recognize their columns
Add support for unknown formats, simply number the columns
Fix infinite loop when previewing data with less rows than limit
Show error when trying to do interpolation with dataset without
timetamps
Show error when trying to plot Time series with dataset without
timestamps
Well.. just show an error when a dataset does not have requested
column..
Only show Event rate variable if data has timestamps
Add comments to some parts of the code
Fix issue with wrong min/max for histograms causing infinite loops
Fix parse_csv, check if the parsed float matches original,
then use the parseFloat value, otherwise the original (for date and
time columns).
Change download arrow to text ‘get csv’
Make dataset_list use full width
Make plot controls and variable lists use full width
Update docs for using local data
Add screenshots to the docs
Merge branch 'local-files'

Add support for loading local csv files