Fixes building non-default parameter sets (#24)

[BradyPlanden]

This PR applies the following:

• Adds / fixes functionality for non-default pybamm parameter sets when building pybop.model()
• Adds pytest parameterization to the test_parameterisation.py class. This runs each parameterisation test across three different SOC points: [0.3, 0.5, 0.8]
• Shrinks the test_examples.py runtime by applying a tighter optimization end condition to examples/scripts/mle.py
• Adds argument for init_soc functionality to model.predict()

[codecov]

Codecov Report

Merging #71 (a6d00e8) into develop (490b86a) will increase coverage by 1.39%.
The diff coverage is 97.61%.

@@             Coverage Diff             @@
##           develop      #71      +/-   ##
===========================================
+ Coverage    90.21%   91.60%   +1.39%     
===========================================
  Files           29       29              
  Lines          756      774      +18     
===========================================
+ Hits           682      709      +27     
+ Misses          74       65       -9     

Files	Coverage Δ
examples/scripts/rmse_estimation.py	100.00% <100.00%> (ø)
pybop/models/base_model.py	94.95% <100.00%> (+7.56%)	⬆️
pybop/models/lithium_ion/base_echem.py	100.00% <100.00%> (ø)
tests/unit/test_parameterisations.py	98.38% <96.87%> (+0.60%)	⬆️