Add simple github actions CI for gcc and clang (#80)

.github/workflows/cmake-clang.yml

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+##############################################################################
+# GitHub Actions Workflow for volesti to build tests with GCC
+#
+# Copyright (c) 2020 Vissarion Fisikopoulos
+#
+# Licensed under GNU LGPL.3, see LICENCE file
+##############################################################################
+name: clang-test
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    name: ${{ matrix.compilers }}
+    strategy:
+          fail-fast: false
+          matrix:
+            compilers: [clang++-6.0, clang++-7, clang++-8, clang++-9]    
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - run: sudo apt-get update || true;
+             sudo apt-get install clang-6.0 clang-7 clang-8 clang-9 lp-solve; 
+             rm -rf build;
+             mkdir build; 
+             cd build;
+             cmake -D CMAKE_CXX_COMPILER=${{ matrix.compilers }} ../test;
+             make;

.github/workflows/cmake-gcc.yml

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+##############################################################################
+# GitHub Actions Workflow for volesti to build tests with GCC
+#
+# Copyright (c) 2020 Vissarion Fisikopoulos
+#
+# Licensed under GNU LGPL.3, see LICENCE file
+##############################################################################
+name: gcc-test
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    name: ${{ matrix.compilers }}
+    strategy:
+          fail-fast: false
+          matrix:
+            compilers: [g++-4.8, g++-5, g++-6, g++-7, g++-8, g++-9]    
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - run: sudo apt-get update || true;
+             sudo apt-get install ${{ matrix.compilers }} lp-solve; 
+             rm -rf build;
+             mkdir build; 
+             cd build;
+             cmake -D CMAKE_CXX_COMPILER=${{ matrix.compilers }} ../test;
+             make;

.github/workflows/cmake-windows.disactivated

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+##############################################################################
+# GitHub Actions Workflow for volesti to build tests with GCC
+#
+# Copyright (c) 2020 Vissarion Fisikopoulos
+#
+# Licensed under GNU LGPL.3, see LICENCE file
+##############################################################################
+name: windows-test
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    name: windows
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v1
+      - run: mkdir build; 
+             cd build;
+             $url = "https://sourceforge.net/projects/lpsolve/files/lpsolve/5.5.2.5/lp_solve_5.5.2.5_dev_win64.zip/download";
+             $output = "$lp_solve.zip";
+             Invoke-WebRequest -Uri $url -OutFile $output -UserAgent  ([Microsoft.PowerShell.Commands.PSUserAgent]::InternetExplorer);
+             unzip lp_solve.zip;
+             cmake ../test;
+             make;