Compiling with cmake under Ubuntu is literally a torture.

[ClaraSchumann]

I've spent a whole afternoon try compiling these code under Ubuntu 20, with g++ and gcc default to g++-9 and gcc-9, respectively. And following the instructions, I use miniconda to manage the environment. After numerous failures, I successfully compiled the code, without modifications to the CMakeCache.txt. Here are some small tips for later users.

1. If you have eigen, fftw or gdal system-wide, DO NOT INSTALL AGAIN FROM CONDA.
2. Install pybind-11 with conda, along the other packages.
3. If the system cmake and git is too old, feel free to get them from conda.

I believe the official instructions for Ubuntu 18 needs some modification.