Change evaluation domain API for cosets

[andrewmilson]

Description

closes: #88

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• Added two constructors new_subgroup(subgroup_size) and new_coset(coset_size, offset). I kept the old constructor new(num_coeffs) since it contains logic to choose a subgroup size
• Removed cosetFFT / cosetIFFT methods
• Added get_coset(offset) convenience method
• Removed divide_by_vanishing_poly_on_coset_in_place(). It wasn't clear if this method was appropriate anymore given domains can be coset domains.
• Removed generator_inv() method and added offset() and offset_inv() methods
• Moved evaluate_all_lagrange_coefficients() into a default implementation. Is there any reason not to do the same for the vanishing_polynomial and element methods?

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• Wrote unit tests
• Updated relevant documentation in the code
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[Pratyush]

Thanks for the PR! These changes all look great. Left some comments!

[Pratyush]

Regarding moving elements and vanishing_polynomial and evaluate_vanishing_polynomial also into the default impl, I think it makes sense!