Represent filters in xlsx export better

[snotskie]

Currently, unitFilter and friends are Functions. These are tricky to represent in plaintext in a spreadsheet cell in a language-agnostic way. We make do by storing the intermediate representatiom of the Julia function. But that makes importing the filter back in difficult, so from_xlsx hasn't been finished yet

One idea I have is to take a page from python and run SQL against the dataframes instead of using a function-based filter

That would give us a neat, language-agnostic, plaintext rep. The limitations though are:

• requires being familiar with sql
• possible performance drop
• opens security vector through possible injections
• limits functionality to what sql can represent