Implement a fetch function on the Invoice

[knikolla]

One of the steps that Kim has to go through is generating graphs and insights from invoices of previous months. We can easily automate this by fetching the already uploaded invoices based on month and running whatever comparisons we need.

We should therefore be able to do

b_08 = BillableInvoice(invoice_month="2024-08")
b_08.fetch(s3_bucket)
b_07 = BillableInvoice(invoice_month="2024-07")
b_07.fetch(s3_bucket)

And this would give us the invoice as exported for 2024-08 and 2024-07 in the data dataframe of the objects b_. We know where to download from s3 if #91 is implemented.

Eventually, we only really want the singular dataframe (after all processors) to be fetchable (which we will also export as is) as applying the future "views" will get us the invoices we need exactly. (Do not worry about PI invoices)

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THESE TASKS ARE LOW PRIORITY