refactor garmin_fs class (#1203)

refactor garmin_fs class (#1203)

* use container for garmin_fs ilinks.

and leave memory management up to the copy on write container.

* make gmsd const for read only usage.

this prevents the possiblity of detachment of the ilinks list.

* incorporate some variables and functions into garmin_fs_t

* unify garmin category conversions.

* fiddle with test time zone sensitivity.

* add utc option for garmin_txt reader.

* correct sign of adjustment

* update garmin_txt includes.