gr-fec - LDPC: Improving memory management (GSL matrices).
gr-fec - LDPC: Renaming class from ldpc_par_chk_mtrx to ldpc_R_U_mtrx

To indicate that this matrix has a specific format, designed for use with the ldpc_R_U_encoder, renaming this class from ldpc_par_chk_mtrx to ldpc_R_U_mtrx.

Also putting the memory free calls in the correct place in the bit flip decoder.