ENH dwidenoise: Fixed a priori noise level

[Lestropie]

Requested in #3035.

Somewhat related to #3046, where a fixed rank of the output data can be requested.

Here, the pre-existing -noise_in command-line option would be changed to a .type_various(). If it can be interpreted as a floating-point number, then that value should be used as a fixed noise level for all patches.