[On hold] Consistent parsing of "None" from xlsx sheet

[santiagoinfantinom]

There is a string "None" here, as I thought that data in the xlsx would be parsed as 'type(None)'. Refernce in link below:

multicriteria_data[TARIFF] = {}
https://github.com/rl-institut/offgridders/blob/dev/src/B_read_from_files.py#L253

if column_item == TITLE_TIME:
https://github.com/rl-institut/offgridders/blob/dev/src/B_read_from_files.py#L349