Update calibrate.py due to new tax function parameters

[jdebacker]

In Issue #36, @jpycroft notes the following:

File "c:\users\jonat\repos\og-uk\oguk\calibrate.py", line 153, in get_tax_function_parameters
num_etr_params = dict_params["tfunc_etr_params_S"].shape[2]
AttributeError: 'list' object has no attribute 'shape'

After changes in OG-Core (PR #819), the tax parameters are stored in lists, not arrays. Thus a change is needed in calibrate.py in this repo to determine the number of parameters in a different way.