Read Float field as float or decimal (not double) and Add support for Double field

[jbtule]

Floats are stored as strings so they are exact numbers, like decimal type, but maybe if someone types a DBF fields as float they are expecting a float so maybe just return a float? Currently returns a double, would have to to make a breaking-ish change, but it's not right either a float or a decimal make sense.

Double fields are actually stored as doubles, and could return doubles but aren't currently supported.

[jbtule]

It's more complicated that it seamed dbf Double is only support in specific versions of DBFs and "B" potentially conflics with other types of DBFs for blob.