Zernike Coefficients math doesn't check for the first mode

[lbusoni]

Now ZernikeCoefficients allows to sum/subtract 2 ZernikeCoefficients objects, but it simply acts on the underlying _coefficients member without checking if the FIRST_ZERNIKE_MODE is the same for the 2 objects. So it may happen that the operation acts on different modes

[lbusoni]

Also fix the dtype mismatch

In [81]: zc = ZernikeCoefficients(np.zeros(1))

In [82]: zc1 = ZernikeCoefficients(np.arange(10))

In [83]: zc+zc1

UFuncTypeError Traceback (most recent call last)
Cell In[83], line 1
----> 1 zc+zc1

File ~/git/arte/arte/types/zernike_coefficients.py:57, in ZernikeCoefficients.add(self, other)
55 if len(self._coefficients) < len(other._coefficients):
56 c = other._coefficients.copy()
---> 57 c[:len(self._coefficients)] += self._coefficients
58 else:
59 c = self._coefficients.copy()

UFuncTypeError: Cannot cast ufunc 'add' output from dtype('float64') to dtype('int64') with casting rule 'same_kind'