Save data_start even there is no downsampling

[dachengx]

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

Further, fix #867 (comment). As said in that conversation, People might misunderstand the non-downsampled data_start if they are zero.

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: 90.198%. remained the same
when pulling 0cd9988 on save_data_start
into 7bdb03f on master.

[HenningSE]

Thanks, @dachengx we can do it like this! Do you still want to keep the upper limit for the downsampling factor? It was introduced to save disk space and keep data_start for S2s at 0. To avoid further confusion, we could also remove this limit if you want.

[dachengx]

Thanks, @dachengx we can do it like this! Do you still want to keep the upper limit for the downsampling factor? It was introduced to save disk space and keep data_start for S2s at 0. To avoid further confusion, we could also remove this limit if you want.

Maybe yes. I will update #910.