Revisit cell type (maybe non mandatory type hint as an attribute would be sufficent)

[voytech]

CellType as enum has many design flaws:

• it unnecesserily restricts possible type domain (we cannot assume the same type set for unknown exporter implementors)
• it adds complexity of resolving type even if there is no need for it. Type of value does not add any value in case of basic types like boolean, date, string.

CellType could be renamed to TypeHint and modeled as attribute