Remove max_downsample_factor_waveform_start, simplify the saving of data_start

[dachengx]

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

max_downsample_factor_waveform_start is not strictly needed. We can use the following logic: if store_in_data_start is False, do not save the data_start.

Also, change digitize_top to store_data_top, store_in_data_top to store_data_top to unify the variable names.

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]

Changes unknown
when pulling 0e14b54 on data_start_simplify
into ** on master**.

[dachengx]

The incompatible to straxen, shown by the failure of autotests, is because of the change from digitize_top to store_data_top .