Allow lowercase cell reference in GET DIRECT ... function calls

[chrispcampbell]

I received a Vensim model that makes use of GET DIRECT DATA and GET DIRECT CONSTANTS, and the cell argument uses lowercase letters, like this:

	GET DIRECT CONSTANTS('?data', 'sheet' , 'c5' )

This works in Vensim, but does not currently work in SDEverywhere because the underlying library that reads xlsx files is not lenient. If we convert the cell reference to uppercase, then it works fine.

As part of this, we should also handle the case where XLSX.utils.decode_cell fails to decode and throw a more useful error message. In the current implementation, processing continues and you won't know about the failure until the C or WASM compiler reports a compilation error.