[x] Use csvt files to force all columns to be strings (specifically for schedules)
[x] Ignore fields which are blank strings; these should be treated the same as NULL for user ease
Add routines to check for:
[x] Blank fields
[ ] Missing fields which are required - note: this can cause Eppy to fill in these fields with unseen defaults! But cannot just stop Eppy using defaults since this helps with some of the materials.
Implement:
Add routines to check for: