Implemented polynomial baseline correction

Curently lives in plotter_helper.get_poly_baseline.
Also added the tests and an example notebook.
Closes SpectralGroup/spectraplotpy#54.

```python
def get_poly_baseline(spectrum, indices, deg=3):
    """
    Return spectrum, that fits a polynomial of degree :deg: through
    the points given by indices. The polynomial is evaluated at all x
    positions of spectrum. The returned spectrum object can be directly
    substracted from spectrum.

    TODO: what to do with naming, metadata, errors...?
    For now performs a deep copy and appends "Baseline of " to name

    Parameters
    ----------
    spectrum :

    indices :  array of indices or other index object.
    deg  = 3
        The degree of the polynomial.
    """
```