Changed null pointer definition in common.h.

NULL is now (void *)0 in C and default in C++.
Function create_empty_bam_header only accepts unsigned int now (not
negative).

The parameter num_chrom is now unsigned int.
Function create_empty_bam_header return NULL if num_chroms is 0.
Added limits to probability calc in aux_math.

The limits are in recal_config.h
First testing implementation, with check.
Changed parameter size in aux_vector functions to unsigned int.
Merge branch 'testing' into develop
Fix: Added unsigned to size in initialize_vector function (aux_vector).
Aux_vector.c now checks if parameter size in functions is 0.
Implementation for aux_library_testing.