This refactors the preprocessor. The new preprocessor implements the
same scheduling strategy (Modified Round-Robin: give resources according
to round-robin, then group) but should be fairer by changing every time
the RR start UE (i.e., 1st TTI UE 0 starts, 2nd TTI UE 1, etc) when the
UE number is not a divisor/multiple of the RBG size (and the last RBG is
often smaller). This also removes all unneeded code of the preprocessor
such as allocate(), positioning(), etc.

* do not check rlc_ind()
* Simplify retransmission part and remove nb_rb_available var
* Simplify new transmission code
* Simplify fill_DLSCH_dci(), avoid unnecessary copies
* fix: first check No. RBs, then check CCE alloc

Previously, the various rlc_*_ind() functions had a parameter to enable
the MAC to specify the max TB size that a subsequent call to the
corresponding rlc_*_req() (!) function would return. This patch removes
this parameter from the rlc_*_ind() functions and makes the parameter
working that is already in the rlc_*_req() functions.

store_dlsch_buffer() puts the RLC buffer status results in the
UE_TEMPLATE, which is per-CC. Since the DLSCH scheduler works per-CC,
too, we pass the right CC. However, this is not strictly necessary, as
we could do this once before every CC, and it might be updated.

UE_list_t is now used as a separate list. It can be e.g. used in the
scheduler to mark groups of users. The original UE_list_t is renamed to
UE_info_t to reflect that it is a container for information about UEs.

* Simplify store_dlsch_buffer()
* Simplify assign_rbs_required()
* Simplify dlsch_scheduler_pre_processor_partitioning()
* Simplify dlsch_scheduler_pre_processor_accounting() and dlsch_scheduler_pre_processor_accounting_allocate()
* Simplify allocation printing

The original functions used in the fairRR scheduler have been moved to
eNB_scheduler_fairRR

* Remove MIMO_mode_indicator: it is simply not used ATM
* Remove min_rb_unit as parameter: function auto-detects
* Remove commented code
* Remove slice sharing/multiplexing: it is broken
* Remove UE sorting, add UE with add_new_ue() in MAC
  - UE sorting is useless overhead on every iteration, instead it should
    be governed by a scheduling algorithm (e.g., RR or PF)
  - The MAC keeps a UE list and automatically adds a UE
  - UE_list empty, set head to -1
* Remove slice_positioning: the slicing functionality is broken
* Remove unused/untested code

The original reset() function used in fairRR has been moved to
eNB_scheduler_fairRR.c