Track if input files have changed to allow condition to only process data if data are updated

[mstackhouse]

The basic flow would need to look like:

• [ ] Capture inputs files
• [ ] Generate a checksum of the files
• [ ] Cache the checksums someplace readable
• [ ] If subsequent run, compare against pre-existing checksums
• [ ] If input data have updated, flag that the file has changes

[MayaGans]

Function factory?

[MayaGans]

Dev and Prod readers with timestamp checker - function handles logistics but you can adapt and extend.

Add vignette with examples using local data, S3, whatever...?