Correct the definition of tag to align to TIFF v6

[joanma747]

The current definition of "tag":

4.37. tag
In TIFF format, a tag is packet of numerical or ASCII values, which have a numerical "Tag" ID
indicating their information content. [Source: GeoTIFF Format Specification 1.0]

is inconsistent with the TIFF specification version 6 page 7 that says:

"In previous versions of the specification, the term “tag” referred both to the identifying number of a TIFF field and to the entire field. In this version, the term “tag” refers only to the identifying number. The term “field” refers to the entire field, including the value."

I would like to suggest to align both documents. These are the definitions proposed in a BigTIFF draft and that I recommend for GeoTIFF too.

tag::
The identifying number of a field type in the image file directory or the big image file directory.

field::
Each entry on a image file directory or a big image file directory. A field is characterized by a tag numerical value and contains or points to the data.

In addition I cannot find a defintion of "key" and I believe it should be in the list of terms.