Ncflag

- Add reset and init to custom value other than 0.

- Fix method for setting flags. Previous method did not
work properly in some cases, eg. good_quality_qf. Now
clears bits in flag under the mask before setting value.
- Adds a get_mask_for_meaning function.
- Improves documentation and testing.

- Handle flags that are set to fill values, by definition, no flags are
set when the value is filled.

- Change: combine get_flag and get_flags -> get_flag. Pass get_flag
either a list of flag_meanings, or a single flag_meaning string.
- Optimization: exit_on_good option for get_flags_set_at_index. If known
that no other flags can be set on "good" flag and good flag == 0, skip
extra work of searching through all other flags.
- New: get_value_for_meaning function. Use case eg: start by setting all
flags to a "missing_data" value, but avoid hard coding the "missing_data"
value.