Revise variable name "Nitrogen|Cropland Budget|Inputs"

[danielhuppmann]

PR #73 brought in several SDG variables from the NAVIGATE and SHAPE projects, including the variables "Nitrogen|Cropland Budget|Inputs".

Please reconsider whether this variable name makes sense, because it is not clear why nitrogen inputs are considered a budget and it is maybe not required to have a hierarchy level "Inputs" if there is no corresponding output.

FYI @IAMconsortium/common-definitions-land @bs538 @land