Error when intializing FAMC filter without acc or mag samples.

[osskov]

Minor bug but when initializing an FAMC filter with no acc or mag entry, it throws:
AttributeError: 'NoneType' object has no attribute 'copy'

Here is the version of the initialization call in the package:
class FAMC:
def init(self, acc: np.ndarray = None, mag: np.ndarray = None):
self.acc = acc.copy()
self.mag = mag.copy()
self.Q = None
if self.acc is not None and self.mag is not None:
self.Q = self._compute_all()