Mf2

- Found and fixed further errors in the Branin function: now matches paper
- Added MultiFidelityFunction.x_opt attribute for known optimum of function
- Added warnings in MultiFidelityFunction if not l_bound <= x_opt <= u_bound
- Added callable AdjustableMultiFidelityFunction class to serve as factory
instead of the previous 'bare' functions
- Tests:
* 'test_Nd_functions' tests now parameterized using hypothesis' data strategy
- Docs:
* Added mathematical definitions in docstring of every function's file

- Added an 'invert' function to enable switching between minimization and maximization
- Update branin_hf definition: correct constant from 2.25 to 22.5
- Update six_hump_camelback_hf definition:
* include omitted minus sign
* rewrite to speed up calculation

- enabled branch coverage and -Werror flag for tests
- updated naming of functions: no space between function and number
- MultiFidelityFunction stores original name as _name, .name is now a property

- can now install development requirements using `pip install mf2[dev]` notation
- conda references now point to conda-forge channel
- updated array-generation strategy in property-test
- removed obsolete code (himmelblau 3-fidelity)
- Added type hints for adjustable functions
- moved contributions section in Readme to separate CONTRIBUTING.md file
- Tests:
* array-generation in property-test now uses hypothesis' numpy array strategy
* coverage now at 100%: Added tests for remaining uncovered code
* Rewrote regression tests to use pytest-regressions plugin
- JOSS paper:
* Fixed typo
* Rewrote 'statement of need' paragraphs
* Improved closing of the paper: how is package used by authors
- Documentation:
* hardware and software version info now mentioned on performance page
* example-usage separated out into separate file in docs/scripts
* figure of example-usage improved to 600dpi
* page on performance added, script for plots added in docs/scripts, including
original matlab source code for borehole, currin and park91a/b

- Bugfix: corrected bounds for adjustable Trid function

- Removed unused artificialMultiidelity.py
- De-duplicated borehole fidelities
- Added LICENSE file to source distribution
- Documentation:
* updated copyright
* updated develop install instructions
* fixed typo: Burrin -> Currin
* Added 'Adjustable' to titles of adjustable function pages
- JOSS paper:
* Added statement of need paragraph