New command "bcftools convert" with limited functionality
Merged from develop
Improved filtering via -i/-e

- added new functions (strlen,abs,sum)
- variables (ref,alt,n_alt,n_samples)
- allow functions on both INFO and FORMAT fields
- expressions are now case insensitive (with the exception of tag names)
- dropped the unnecessary prefix "%" as conflicts can be avoided by
giving the full prefix (e.g. INFO/QUAL vs QUAL)
Support for AC, MAC, AF, MAF in filtering expressions
annotate: New --rename-chrs option
Test for annotate --rename-chrs
Updated man page for --rename-chrs
man page: Resolved merge conflict
man page: Resolved merge conflict
man page: Resolved merge conflict