Library mode should not be the default compilation mode.

Currently, the default behavior of nvq++ is to compile user code in library mode, i.e., the simple nvq++ <source> -o <exec> will compile <source> in library mode. The problem with this behavior is that this modes allows the user to use the full power of C++ within kernel code---something that is unlikely to be realistic when targeting real quantum devices.

We should create a default emulation target and make the use of the library mode should be a conscious choice by the user, say nvq++ --library-mode <source> -o <exec>, and the compiler should warn the user about its danger: mainly the fact that their kernel might not be able to target real (or emulated) devices.

Start at the nvq++ command-line handling for compilation modes and target selection. Trace how the default mode and library-mode option are chosen, then identify the existing tests or command paths covering these options. Done means ordinary nvq++ compilation uses the emulation target, --library-mode is explicit, and the warning behavior is covered.