make miniapps optional in CMake

[sharkovsky]

Right now all miniapps are built by default, and strange dependencies between the driver and miniapps appear as soon as you try to disable them.
For example, the driver requires the file path.h, which gets generated in the coreneuron_1.0 folder. If you disable that folder from CMake, you get this error

In file included from neuromapp/app/driver.cpp:32:0:
neuromapp/coreneuron_1.0/common/data/helper.h:32:95: fatal error: neuromapp/coreneuron_1.0/common/data/path.h: No such file or directory

We should make the miniapps optional, and independent from one another.