Fix apr

[gerardhros]

Version 0.5.1 2023-05-09

Fixed

• NAm and SAm as abbreviation for North and South America causes errors due to confusion with NA, issue sptf_countries$continent_code for North America being "NA" creates missing values #3
• replace assert_int with assert_integer for years in sptf_cdec1, issue sptf_cdec2 only accepts one year? #26
• ensure correct CN ratios in sptf_cdec1, issue sptf_cdec2 only accepts one year? #26

Changed

• all elements for A_CACO3_MI replaced with A_CACO3_IF, issue replace all A_CACO3_MI with A_CACO3_IF #30

Added

• options for parameter B_SOILCLASS_USDA in sptf_parameters, issue add possible options #23
• add ptfs cec75, cec76 and cec77