Matplot++ Install for Default Ubuntu 18 w/o new CMake

[acxz]

Hello everyone, I worked on a patchset to get Matplot++ to compile natively on Ubuntu 18 without manually installing CMake 3.14+ but to instead use 3.10.

CPM gets disabled as it requires features from 3.14, but you can still follow the other build/install instructions.

Here is the associated PR: #88

Hope someone finds this useful!