Remove or Restructucture "data_path" option and use in config about flepimop HOT 3 CLOSED

This issue requires a bit more discussion. The main discussion items are:

• I suggest we don't call this "data" ... since "data" makes people think about actual disease observations. Can we call this "model_input" for example?
• Are we sure we want a single place to look for all this info? Maybe every file should be specified by the full relative fill path. That way, people can organize files as they wish, and if they have unique directories storing different information, perhaps because it's output of another simulation, they have that option.

Some related issues:

• Data_path is not where the config looks for the initial conditions, seeding file, or parameter timeseries. Are there other config inputs where values are looked up in a separate file?
• It is actually a bit confusing for which seeding/IC options you need to specify a complete file path vs just file name within "data_path". For example, where do FolderDraw methods look?

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Relatedly:
#78

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Decided instead to remove the data_path config input. Instead, specify the full (relative) file path for every input file. We will set the norm of storing all model input files in a model_input directory. Need to update flepimop core code to look for geodata in a path that does not involve appending the "data_path" directory onto the geodata and mobility data file names

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