Support BINTABLE boolean NULL values

[fxpineau]

Here in the code, the boolean parsing is incorrect.
It should be something like byte == b'T' instead of byte != 0.
And byte == 0 is used for a NULL value, see section 7.3.3.1 of the FITS document.
I image two solutions:

• add a Null to the DataValue enum
• replace bool by Option<bool> for value in DataValue::Logical

Which one do you prefer (I have not looked enough at fitsrs to have an opinion)?

P.S: in AT2S I use both a Field::Boolean(bool) and Field::NullableBoolean(Option<bool>) so I then to favor the second option in the above list.