Update amplitude masking for Channel template

[rmsare]

Currently the Channel class is a duplicate of the Ricker wavelet. This is fine, but the channels this template is meant to detect will have positive amplitude only when convolved with this template function, so masking out negative amplitudes is desirable.

Right now the match function returns positive and negative amplitudes by default, but this can be changed by

• Implementing a MaskedTemplateMixin with a get_amplitude_mask method
• Masking with this method in core.match_template in a similar way to get_err_mask