The signature of `load_single_emeg` is very convoluted

[caiw]

Could do with a simplification or splitting into multiple functions for all the different modes:

• Load sensor data
• Load sensor data and apply precomputed inverse solution and precomputed morph
• Load sensor data and apply precomputed premorphed inverse solution

At the moment it's very unclear from within the function what path will be taken given the state of the input arguments.