[electrophysiology_browser] Revised EEG filters + Store reference fix (…

…#9038)

This does the following:

- Uses a revised set of coefficients values for the filters, depending on the recording's sampling frequency
- Uses multiple store references when there are multiple recordings. It would previously get overwritten.


The following script was used to generate the coefficients:

```
samp = 512; % Change to target frequency
order = 3;

low_pass_items = [15 20 30 40 60];
high_pass_items = [0.5 1 5 10];

for i=1:length(low_pass_items)
        disp(strcat(num2str(low_pass_items(i)), ' low pass'));
        [b, a] = butter(order-1, (low_pass_items(i) / samp), 'low') % argument is (order - 1)
end

for i=1:length(high_pass_items)
        disp(strcat(num2str(high_pass_items(i)), ' high pass'));
        [b, a] = butter(order-1, (high_pass_items(i) / samp), 'high') % argument is (order - 1)
end
```