Improve flexibility of the precision parameter in the CWT implementation.

[meiyasan]

Hello @rgommers ,

I had many issues in controlling the precision of the wavelet (def wavefun()). This issue has been mentioned several times already, but never committed. I think this is a rather important fix allowing for more flexibility in playing with CWT.

So please find the minimal modification necessary allowing for better precision and removing artifacts in the low-frequency region (high scale values).

Please consider if possible, this is important parameter to adjust. In the current master, the value is hardcoded in _cwt.py to precision=10, this is not so good.