compile issues in Ubuntu

[cheshmi]

I tried to install IPC on Ubuntu 16.04 and I got couple of compile errors. I fixed them and sent a PR. I changed some lines in src/CollisionObject/CollisionObject.h and src/CollisionObject/HalfSpace.cpp.
Here is what I did to install the modified code:
Pre-requisites (in addition to what the repo says):

• GMP LIB: sudo apt install libgmp-dev
• Also, you will need gcc 7 or later. I installed it like (sudo add-apt-repository ppa:jonathonf/gcc-7.1
  sudo apt-get update
  sudo apt-get install gcc-7 g++-7)

Then you can use the following:
mkdir build
cd build
cmake -DCMAKE_CXX_COMPILER=g++-7 -DCMAKE_CC_COMPILER=gcc-7 -DCMAKE_PREFIX_PATH=/home/kazem/programs/SuiteSparse/ -DCMAKE_BUILD_TYPE=Release ..
make -j4

I could run the first tutorial.

[liminchen]

Thank you Kazem! This is helpful. I've updated readme and merged your pull request.