Remove deprecation of "missing_value" attribute

[caron]

CF incorrectly deprecates the "missing_value" attribute due to a misunderstanding.

All changes refer to section 2.5.1:

1. Change the first paragraph from:

"The NUG conventions (NUG section 8.1) provide the _FillValue, valid_min, valid_max, and valid_range attributes to indicate missing data."

to:

"The NUG conventions (NUG section 8.1) provide the _FillValue, missing_value, valid_min, valid_max, and valid_range attributes to indicate missing data."

2. remove the word "strongly" from this sentence:

"If only one missing value is needed for a variable then we strongly recommend that this value be specified using the _FillValue attribute."

3. Remove this paragraph:

"The missing_value attribute is considered deprecated by the NUG and we do not recommend its use. However for backwards compatibility with COARDS this standard continues to recognize the use of the missing_value attribute to indicate undefined or missing data."

4. Change the first sentence of the last paragraph from:

"The missing values of a variable with scale_factor and/or add_offset attributes (see section Section 8.1, “Packed Data”) are interpreted relative to the variable's external values, i.e., the values stored in the netCDF file. "

to:

"The missing values of a variable with scale_factor and/or add_offset attributes (see section Section 8.1, “Packed Data”) are interpreted relative to the variable's external values ( a.k.a. the packed values, the raw values, the values stored in the netCDF file), not the values that result after the scale and offset is applied."