DIG-1267, DIG-1268

[daisieh]

DIG-1267: Making the template even more fulsome and self-explanatory:

• template generation now includes DONOR.INDEX at the front, so that the mapper can see that all entries are nested inside a DonorWithClinicalData object
• each mapping defaults to a SHEET_NAME.field_name format, with the probable sheet name filled in. Mappers can do a global find/replace to the actual name of the sheet.

DIG-1268: Making the distinction between mapping methods and internal (or convenience) methods more clear in mappings.py

• prepended an underscore to the names of internal methods, to make it clearer that mappers shouldn't use them in their mappings.
• also did a bonus fix for date parsing: don't try to convert strings with no numerals in them to dates.

Test by generating a template with python generate_schema.py. You should see that the indexed_on defaults should map to a particular sheet's value and that sheet/value combination should make sense for that nested object.

There should be no visible changes to the other fix: CSVConvert should work as before, maybe a little faster.

[kcranston]

Thanks for the improvements! I expect documentation changes in the readme and / or mapping_functions.md to go along with this (with some of the explanation you included with this PR).