Deletion of API examples and `copy-examples`

@mesnardo Why do the API examples have to be deleted after installation?

Line 17 in ffdbe25

rm -rf $(prefix)/examples/api_examples ; \

And is the instruction for copying examples valid?

Lines 251 to 260 in a657821

    
           ## PetIBM examples 
        
           We provide some examples! 
        
           Input files are located in `$PETIBM_DIR`, but can be copied to another directory with 
        
           ```shell 
        
           make copy-examples EXAMPLES_DIR=<directory> 
        
           ``` 
        
           `EXAMPLES_DIR` is optional and the default directory is the folder `examples` in the top build directory.

Hey @piyueh , they are probably removed because they are not compiled with the root make all. To compile them, we need to change dir to the api example folder and run make. We have instructions in the README file of each api example.

And you are right, the copy-examples recipe does not work anymore. Now examples are copied into the installation directory with make install. (Feel free to update to docs.)

resolved in #164

	## PetIBM examples

	We provide some examples!
	Input files are located in `$PETIBM_DIR`, but can be copied to another directory with

	```shell
	make copy-examples EXAMPLES_DIR=<directory>
	```

	`EXAMPLES_DIR` is optional and the default directory is the folder `examples` in the top build directory.