integrate GRIB level type and level value functions.

[perillaroc]

level type and level value are connected. So convert functions should be combined together.

Current functions:

• reki.format.grib.common
  • fix_level_type
• reki.format.grib.cfgrib._util
  • _fill_level_type: add typeOfLevel to filter_by_keys
  • _fill_level_value: add level to filter_by_keys
• reki.format.grib.eccodes
  • _level
    • _fix_level: convert pl, ml and sfc, and return level_type and level_dim's name
  • _util
    • _check_level_type:
    • _check_level_value: calculate pl's level, deal with first_level and second_level in dict.