fix: support serialization with pickle

[jvdd]

Currently none of the Rust downsamplers can be serialized with pickle as pickle throws a TypError when serializing a module

• add support for pickle
• add tests for this

Minimal code example ⬇️

In [1]: from tsdownsample import MinMaxDownsampler

In [2]: d = MinMaxDownsampler()

In [3]: import pickle

In [4]: pickle.dump(d, open('test.pkl', 'wb'))
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[4], line 1
----> 1 pickle.dump(d, open('test.pkl', 'wb'))

TypeError: cannot pickle 'module' object

---

Related: PR #34 added support + tests for deepcopy