CPM instead of FetchContent and ccache for ultra fast build

[dcoeurjo]

PR Description

CPM

Fetching deps is now done with CPM (faster than FetchContent). Furthermore, if you add

 export CPM_SOURCE_CACHE=$HOME/.cache/CPM

to your .bashrc ou .zshrc, CPM will cache the downloads.

CCache

When you have installed ccache on your system, the USE_CCACHE=yes cmake option enables it for the DGtal build. The first build has a small overhead but the other builds will be extremely fast (when iterating with make clean; make).

For instance (with examples):

• with ccache, after the first build: make 11,66s user 8,42s system 102% cpu 19,618 total
• without : make 287,18s user 20,38s system 99% cpu 5:08,66 total 

cmake-ccache enabled using the CPM.cmake framework (more to come).

Checklist

• Unit-test of your feature with Catch.
• Doxygen documentation of the code completed (classes, methods, types, members...)
• Documentation module page added or updated.
• New entry in the ChangeLog.md added.
• No warning raised in Debug mode.
• All continuous integration tests pass (Github Actions & appveyor)

[dcoeurjo]

Still failing because of PythonTest but all c++ builds are fine. Ready to review.

[dcoeurjo]

Ping @kerautret @JacquesOlivierLachaud could please have a quick look?

[dcoeurjo]

Ok selfmerging