Fixes mac sched tun

A number of fixes for the MAC scheduler and for multiple UEs (notably L2 simulator):

• bugfix in the default DL round robin scheduler which should prevent scheduling issues with >10 UEs
• fix for the default UL round robin scheduler which prevents that any UE ever gets more than 48 RBs, which seems to be a bug in the PHY (but I am not sure)
• bugfix in the static slicing algorithm
• refactoring in the scheduler
• removes some mutexes in the phy_stub_UE.c file: we nowadays do not run it with multiple threads, so we should not need these mutexes
• lowers log level of nFAPI in L2 simulator
• removes UESIM_EXPANSION define: if enabled, the lte-softmodem does not compile. Its purpose (apart from increasing the number of UEs, which can be achieved differently), is to multiplex multiple UEs on a single TUN interface. This only works under certain circumstances (like a linear relation of PDCP ID <> IP address, highest byte). Instead, this MR simply opens the right amount of TUN interfaces (can be anything, I tested 128 TUN interfaces) and does not do such multiplexing. Thus, any IP address might be allocated to a UE, and is correctly represented in the host machine.