hampel_filter filtsize doesn't work as expected/intended.

Two issues:

The documentation for the hampel_filter function states:
```
filtsize : int
    the filter size expressed the number of datapoints
    taken surrounding the analysed datapoint. a filtsize
    of 6 means three datapoints on each side are taken.
    total filtersize is thus filtsize + 1 (datapoint evaluated)
```
however, as currently implemented the slices will not be filtesize + 1, it will be exactly filtsize. e.g. for filtsize = 6, the following line:
```
dataslice = output[i - onesided_filt : i + onesided_filt]
```
on the first iteration of the loop (i = onesided_filt -> i = 3) will evaluate to dataslice = output[0:6]. Python is exclusive on the end of the slice, so you will only get 6 data points in the slice. I believe the code should read:
```
dataslice = output[i - onesided_filt:i + onesided_filt + 1]
```
behavior for odd numbered filtsize is not well defined. Python's // operator will round down for odd numbers (e.g. 7//2 will result in 3). This means a filtsize = 7 is actually functionally the same as filtsize = 6. If I understand correctly an odd-numbered filtsize isn't really intended to be possible, so I'd suggest adding a guard clause at the top of the function to prevent this, as well as updating the docs.

Thanks!

paulvangentcom / heartrate_analysis_python