Fix #48 : Removing unused functions and class methods

Removes the following functions which are not used/required anywhere else in the package.

minterpy/utils.py

• report_error

minterpy/jit_compiled_utils.py

• lp_norm_for_exponents
• compute_grad_c2c
• compute_grad_x2c
• insert_single_index_numba

minterpy/core/utils.py

• get_partial_derivatives
• sort_lexicographically

minterpy/core/grid.py

• Grid.enlarge
• remap_indices

minterpy/core/verification.py

• check_is_square

minterpy/polynomials/lagrange_polynomial.py

• _union_of_exponents