debug_logger: add log_debug to logger

[jvdp1]

Following the discussion in #254, I propose to add a subroutine call log_debug with the following API:

Description

Writes the string message to self % log_units with optional additional text.

Syntax

call self % log_debug( message [, module, procedure ] )

Behavior

If time stamps are active, a time stamp is written, followed
by module and procedure if present, and then
message is written with the prefix 'DEBUG: '.

QUESTION: Is this API OK? Or should it be more similar to the API of log_error with stat and errmsg?

[wclodius2]

I think the API is good. In particular I would not add a stat or errmsg.

[milancurcic]

Looks okay to me as well.

[jvdp1]

Thank you for the review. I'll merge it and submit the next PR mentioned in #254