Do not change channel when sort_by_time

[dachengx]

What is the problem / what does the code in this PR do

'channel' was changed in

strax/strax/processing/general.py

Lines 21 to 24 in b0e7fdc

	min_channel = x['channel'].min()
	channel = x['channel']
	if min_channel < 0:
	channel -= min_channel

.

So use x['channel'].copy() to make a copy of x['channel'] to avoid this bug.

Can you briefly describe how it works?

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• Update the docstring(s)
• Update the documentation
• Tests to check the (new) code is working as desired.
• Does it solve one of the open issues on github?

Please make sure that all automated tests have passed before asking for a review (you can save the PR as a draft otherwise).

[coveralls]

Coverage: 92.19% (-0.02%) from 92.209% when pulling 5008aba on sort_ch_unchange into 38d24ae on master.