V03 make *filetype* an sr3 extra feature, because of dependency issues.

[petersilva]

As explored in #721: There is a new feature in 3.0.42, where the content-type header of each file is included in the message... so people can know the filetype before downloading. It is a mandatory field in the WMO messages, so needed the functionality to be able to export cleanly, and itś a good thing anyways, so added it to base message format.

That required an additional dependency ¨python-magic" It turns out this dependency is troublesome. The debian packaged python packege is python3-magic, the pip install package name is python-magic but that only works on linux. on windows you need to install python-magic-bin. On linux, the debian package has a dependency on libmagic1, but if you install with pip, it won´t pull in the dep... and containers seem to be missing it. So in some cases you need to install libmagic1 by hand separately. A bit of a mess.

Already have a mechanism for dealing with troublesome or fairly optional dependencies, in python they are called extras. but that only covers installation. There is also code built into sr3 to check for presence of extra libraries and only use it if it is present. so ´filetype' is now an extra feature. In this case, it just degrades to omitting the header in the messages it posts.

There is also some missing documentation in the install process, because the amqp library is now an optional extra, and nobody changed the docs... to install you need to install a message passing library as well as the package now, So added that to the documentation.

(some people want to be able to do minimal installs with mqtt, and resented having to have amqp on the machine.)