write_simspin_FITS fails when parameter input_simspin_file is an R object

[kateharborne]

Raised by @doctorcbpower: write_simspin_FITS() function fails when parameter input_simspin_file is an R object, and does not give a meaningful error:

Error in Rfits_write_key(filename = filename, keyname = keynames[i], keyvalue = keyvalues[[i]],  : 
  keyvalue must be length 1

1. Not immediately clear that the parameter SHOULDN'T be an R object.
2. No useful error to allow a user to fix this...

Probably requires:

• Clearer parameter name (i.e. input_simspin_file_path)
• Meaningful error if an R object is provided.