[Opm] Updated compiler and library version requirements

[Atgeirr Rasmussen]

Dear OPM community,

At the recent OPM meeting, it was decided to require newer versions of some libraries and compilers for building OPM Flow going forward. The new requirements are:

 - C++ compiler with C++17 support (gcc minimum version 7).
 - Dune version 2.6 or newer.

These requirements are easy to satisfy on newer systems such as Ubuntu 18.04. In particular, Dune 2.6 is provided for Ubuntu 18.04 through our package archive (ppa), which can be added as follows if you did not do so already:

sudo apt-add-repository ppa:opm/ppa
sudo apt-get update

For older systems such as Ubuntu 16.04 it will still be possible to install binaries of existing versions of OPM Flow (up to version 2019.10), but if you need newer features you will need to upgrade.

Build instructions will be updated shortly to reflect this.

Atgeirr and Arne Morten