Speed up recursions

[BalzaniEdoardo]

In this PR:

• New accept integer for __mul__: basis * 3 will be equivalent to basis + basis+basis
• Add an __rmul__ enabling right multiplicationn * basis
• Improved the efficiency of __pow__ by bisect multiply (log2(n) multiplication, less deep copy)
• Add a mechanism that do not perform a deep-copy when cloning a bases

Old clone performance

bas = nmo.basis.BSplineEval(5)**500
%timeit bas.__sklearn_clone__()
2.47 s ± 107 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

New clone performance

bas = nmo.basis.BSplineEval(5)**500
%timeit bas.__sklearn_clone__()
10.9 ms ± 22 μs per loop (mean ± std. dev. of 7 runs, 100 loops each)

Old Power Performance

bas = nmo.basis.BSplineEval(5)
%timeit bas ** 500
2.49 s ± 76.7 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

New Power Performance

bas = nmo.basis.BSplineEval(5)
%timeit bas ** 500
8.46 ms ± 79.3 μs per loop (mean ± std. dev. of 7 runs, 100 loops each)