Cmake examples

[mkirilin]

This PR simplifies and unifies examples/CMakeLists.txt file.
In addition, it make examples build by default.

[cburstedde]

Thanks! is this as much as possible analogous to the libsc code for the examples?

[mkirilin]

It is a good balance between simplicity and similarity with the libsc, considering the peculiarities of p4est (e.g. 2 or 3 dimensions, etc.)

[cburstedde]

It is a good balance between simplicity and similarity with the libsc, considering the peculiarities of p4est (e.g. 2 or 3 dimensions, etc.)

Cool! CMake has its own way of defining enable p4est, p6est, p8est. Do the examples compile in all combinations?

[mkirilin]

Cool! CMake has its own way of defining enable p4est, p6est, p8est. Do the
examples compile in all combinations?

Yes, I've tried all the 4 combinations of p6est / p8est and it works well.