Add a function to directly load file from strax folder

[dachengx]

Using only information inside the folder

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

Sometimes we want to directly load the strax file in a folder, without initializing plugins or context.

Can you briefly describe how it works?

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

results = strax.dry_load_files(dirname)

this will load all files inside a folder.

results = strax.dry_load_files(dirname, 1)

this will load the 2nd chunk inside a folder.

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: 91.501% (+0.05%) from 91.452% when pulling dcf30e7fa8a8aee88beb16b3b625ce522ea9de87 on dry_load_files into 1a935264b82ab463a972a5ffc66d04a1c6ed5663 on master.