gsd/develop: handle blocked data structures in init and finalize routines, remove pset logic

[climbfuji]

Changes in this PR:

• remove pset logic from scripts/ccpp_prebuild.py and scripts/mkdoc.py
• bugfixes for generating list of requested versus provided variables (CCPP_VARIABLES_*.pdf)
• improved error detection for optional arguments
• add additional unit conversions K<->C and g kg-1 <-> kg kg-1
• handling of blocked data structures in _init and _finalize routines, requires using the dimension (existing) and active (new; name subject to change) metadata attributes
• remove support for old metadata standard
• add legacy extension to handle inconsistent use of horizontal_dimension versus horizontal_loop_extent (otherwise a huge amount of metadata-only changes would have to be made; this should be done in a separate follow-up PR)
• support for handling conditionally allocated/used variables when converting blocked data structures or units

Associated PRs:

#4
NOAA-GSL/ccpp-physics#29
NOAA-GSL/fv3atm#30
NOAA-GSL/ufs-weather-model#23

For regression testing information, see NOAA-GSL/ufs-weather-model#23.