nematodes in development

[gerardhros]

since nematodes are measured via separate labs, and they are only applicable in a few cropping systems, its better to make this optional in the OBI package.
needs rethinking of the overall package structure given OBI wrapper.

[BrentWHR]

ind_nematodes() calculates I_B_NEM all by its self. I'm not sure the structure of the wrapper needs to change when including nematodes is optional. If one wants to include nematodes we could calculate I_B_NEM and feed it into the obic_field() wrapper.

Some nematodes from other labs still need to be added combined with some more checks and perhaps warnings to identify analysis/labs that we haven't encounterd before but this are seperate issues from the one raised in your comment.