add Text Area Description to header reader/writer

[floriandeboissieu]

Hi Jean-Romain,
In the current PR, I propose an implementation of the VLR Text Area Description for header reader/writer.
Cheers

[Jean-Romain]

Thank you, I indeed missed this part of the specs. Can you make a clean PR. It seems that the diff is reporting much more code modifications than you actually did and it is thus hard to see what you actually changed. Thank you.

[floriandeboissieu]

The large change is due to an excessive indentation I corrected (l270-l409 in the original readheader.cpp file). Unfortunately, github diff does not report it well... but according to https://github.blog/2011-10-21-github-secrets/
adding ?w=1 to the commit url would show that it is only space changes.

In order to make all that clean, i.e. separate the real changes from the indentation changes, I made another PR #64 as suggested.