Default Plot, Targets, And Selections

[O957]

For the full scope of this PR, please refer to issue #75 .

Specifically, this PR adds:

• A button called All underneath the model selectbox that selects all models.
• A button called None underneath the model selectbox that de-selects all models.
• The default for the model selectbox set to all models.
• Edits to the Next and Previous buttons to abide better with PEP-008, i.e. "_single_leading_underscore: weak “internal use” indicator."
• Target selection defaults to the first target, alphabetically.
• [-] A function called _cut_observed_to_forecast that truncates the observations to be extra_weeks longer than the minimum forecast date.
• The default for reference to be the most recent reference date.

Somewhat out-of-scope but still useful, this PR also adds:

• Two functions target_selection_ui and model_selection_ui in place of model_and_target_selection_ui, to keep the code more organized and atomic; corresponding updates have been made in main.
• Two functions location_selection_ui and reference_date_selection_ui in place of location_and_reference_data_ui, again to keep the code more organized and atomic; corresponding updates have been made in main.

[O957]

Issue at present where the defaults for the models and reference date to do not work (as in, they are not selected) if the forecast file is uploaded after the observations file. If the forecast file is uploaded alone everything works as intended.

Fixed via b1da630

[damonbayer]

Thanks @AFg6K7h4fhy2!