APT Demod: Update to latest aptdec.

[srcejon]

Updates to use the latest version of aptdec. This will require rebuilding of aptdec library dependency as the API has changed (as per Linux build instructions).

Removes obsolete zenith variable and adds support for visible image calibration.

Fixes #1422

[srcejon]

The docker build looks like it has failed due to using the old version of aptdec.

[f4exb]

It should pick up the latest shouldn't it? https://github.com/f4exb/sdrangel/blob/master/external/CMakeLists.txt#L773

[srcejon]

It should do, and the other two were OK. Is it possible to run it again?

[f4exb]

It is an issue with Appveyor insisting to use a cached version:

#23 [aptdec 2/2] RUN git clone https://github.com/srcejon/aptdec.git     && cd aptdec     && git checkout libaptdec     && mkdir build; cd build     && cmake -Wno-dev -DCMAKE_INSTALL_PREFIX=/opt/install/aptdec ..     && make -j8 install
#23 sha256:700ccc8b5e68073f4a5922342f140d0fc9eba212dd1750f4c2c5fd2bf1407dd2
#23 CACHED

I does not make any sense and I don´t know how to tell it not to use a cache,

[f4exb]

I think I've found: https://www.appveyor.com/docs/build-cache/#cleaning-up-cache