[Feature] Add simple functions to query language results

I'll try to explain by example:

I have invoices that I can submit to my insurance to get them refunded. Each invoice has amongst others the following custom fields:

- `submitted_to_insurance` (boolean, if the invoice is submitted for to the insurance already)
- `invoice_total` (numeric, the amount of money due to be paid)

The insurance wants invoices to be submitted in batches. Submissions must total at least 100,00€.

I can easily find the invoices that I need to remind myself to submit with the following query:
`(& f:eingereicht_beihilfe="false")`

However, this is annoying if the grand total of the unsubmitted invoices does not exceed 100,00€ yet. I'd love to be able to only show the results of the above query, if the sum of their custom fields `invoice_total` is greater than 100,00€.

Some functions would be needed, like:
- `sum` (for the above example)
- `minimum`
- `maximum`
- `mean`
- `count`

The results of these functions are already displayed in the UI header.

Would this be possible somehow?
Thanks for looking into this and this great software!