Update aocommon and add Dockerfile for gcc11 test

docker/Dockerfile-Ubuntu18-gcc-11

+# Copyright (C) 2020 ASTRON (Netherlands Institute for Radio Astronomy)
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+# This is a Dockerfile specifically to test compilation using
+# gcc 11, which is as of writing (Aug 2021) not standard yet, but is more
+# strict than gcc 10.
+
+FROM ubuntu:18.04
+
+RUN export DEBIAN_FRONTEND=noninteractive && apt-get update && \
+    apt-get install -y \
+    build-essential \
+    casacore-data casacore-dev \
+    cmake \
+    libblas-dev liblapack-dev \
+    libpython3-dev \
+    libboost-filesystem-dev \
+    libboost-system-dev \
+    libboost-test-dev \
+    libcfitsio-dev \
+    libfftw3-dev \
+    libgsl-dev \
+    libhdf5-serial-dev \
+    pkg-config \
+    python3 \
+    python3-distutils \
+    wget \
+    software-properties-common && \
+  echo -ne "\n \n"| add-apt-repository ppa:ubuntu-toolchain-r/test && \
+  apt-get update -qq && \
+  apt-get install -y gcc-11 g++-11 gfortran-11
+
+ADD . /src
+WORKDIR /src
+
+RUN \
+  mkdir /build && \
+  cd /build && \
+  CC=/usr/bin/gcc-11 CXX=/usr/bin/g++-11 cmake ../src && \
+  make -j`nproc` && \
+  make install

aocommon @ aea90ad3

-Subproject commit 088140529903105dba98202be8dc6c72d28b1b8e
+Subproject commit aea90ad3b9e0e67e618cf02cc2d38a1275317fbf