for example 20:30000000-32000000 can be now given also as 20:3e7-3.2e7.
Replaced atoi parameter parsing with strtol to recognised user errors.

Parse digits by hand in hts_parse_decimal(), allowing integer parsing
of integer or scientific notation possibly with commas (before the decimal
point), and allowing possible future expansion to "3Mbp" and the like.

Add doxygen-style documentation, noting that warnings will be produced
for invalid input.

Rewrite hts_parse_reg() to take advantage of hts_parse_decimal()'s comma
abilities and validation.  Return NULL (instead of suggesting the sequence
name is the whole region string) when the region cannot be parsed.

(Changing the "*beg > *end" test to ">=" fixes the off-by-1 aspect
of samtools/samtools#353; returning NULL provides the opportunity to
improve the error message.)