Clean up of documentation

docs/basics-of-parameter-estimation.txt

docs/compact-binary-coalescence-parameter-estimation.txt

docs/installation.txt

docs/likelihood.txt

docs/prior.txt

docs/tupak-output.txt

docs/writing-documentation.txt

examples/other_examples/linear_regression.py

examples/other_examples/linear_regression_unknown_noise.py

tupak/__init__.py

tupak/core/__init__.py

tupak/core/likelihood.py

tupak/core/prior.py

tupak/core/sampler.py