Get obs update

[EricSaboya]

The following updates that primarily relate to get_obs.py have been made:

get_obs.py

• input option for calibration scale (update propagates to hbmcmc.py and .ini files)
• input option for measurement data quality (update propagates to hbmcmc.py and .ini files)
• block for creating lists of None for args that specify just None (helps with for loop cycling)
• Added species option for retrieving footprints so CO2 footprints are specified
• Streamlined the model_scenario part of the code so only 1 model scenario object is created instead of N
• added option for calculating mf_mod for different fluxes with specification of whether it is mf_mod or mf_mod_high_res
• Fixes issue #52 from Stephen P. if no site data found process now exits

hbmcmc.py

• Changed default prior PDFs to something more sensible
• added new args and added documentation to function
• Tidied up docs and comments in file

inversion_pymc

• Inversion does not run with save_trace block. Had to comment out in order for example .ini to run

ini files

• updated example files so they actually run on BP
• added new variables
• tidied up commenting etc.

NB. when using fp_model=NAME as an arg, fps are not retrieved. Changed default to None.