Add .txt to ADDA output files

[alkichigin]

Is your feature request related to a problem? Please describe.

ADDA output files have no filename extension, although (for now) they are always txt-files.

Describe the solution you'd like

Add ".txt" at the end of output files

[myurkin]

This is somewhat related to #183 . Anyway, the next step is to understand what is the standard (or several standards) existing for file extensions. While it is clear that extension is universally used on Windows, I thought before, that for Unix users absent file extension is OK (and is commonly assumed to denote text files). If however, the latter opinion is universally outdated, it will give us motivation to make some changes.

Moreover, if we decide to add some extension to the output files, .txt is not the only option. Consider, e.g., .dat that is already used for some of input files by ADDA.

[alkichigin]

*.txt is the only file extension that is straightforwardly opens as plain-text file in all of the operating systems. I suggest switching from extension-less to *.txt file output until this issue is resolved thoroughly.