tesseract:: retry OCR with different config sets

[GerHobbelt]

alternative and more powerful implementation of the current (hacky) retry_config argument:

Purpose:

• when OCR action fails, retry with a different parameter set by specifying a different set of config files to load (this is tesseract's way to mix the parameters)
• continue trying until the set alternatives run out or page OCR is reported as successful

[GerHobbelt]

The idea here being:

using a tesseract parameter to point at a prescription file which lists alternative runs to try, one set per line, e.g.

# retrial prescription
# 
#   can specify simple parameters directly by assignment; other words on the line are config file references
#

lang=eng    psm=3     config_A
lang=eng+lat+fra+deu   psm=3     config_A
psm=1    config_debug_all
psm=7
psm=11
psm=13

NOTE: I'm still pondering whether a subsequent line/set should augment/replace a previous line's settings, i.e. not revert to factory default before processing the line, OR start clean every time (reset to factory default) on each round. The latter is more obvious, while the former makes for tighter set lists as we only need to specify what needs tweaking for the new round... 🤔

[GerHobbelt]

Incidentally, this is another argument why I want every option in tesseract to be steerable/configurable via parameter: using CLI and direct C++ API interfacing should be equally powerful & easy at the same time.