Create 'custom' crop type

[cpearson1]

Adding crops not currently included in the ETDemands model requires multiple steps and script changes. Users often adjust existing crops to meet their needs and file changes can be lost during
sharing and overwriting steps.

It might be useful to add a 'Custom' crop type that can be edited (i.e. CropParams.txt/CropCeofs.txt) by users as needed without script changes. This will stop adjustment established crops/curves and allow for easier implementation of crops/curves not included in the model.

Users can request 'official' addition of crop types by providing the necessary input information.