Fix resolving --strict value

[karol-brejna-i]

Fixes #21

Please, note the following lines:
https://github.com/vitogit/pgn-tactics-generator/compare/master...karol-brejna-i:fix-strict?expand=1#diff-e31f1011e582ea3d502823f13a4a64fdef87e41ed47178a0a6e497df1402a27cR139-R140

There were cases where the number of analyzed legals was 2. In that case

self.analysed_legals[move_number].evaluation.cp

failed with IndexError

I proposed an addition condition (as above), but don't really know the intent of that piece of code.

[vitogit]

@karol-brejna-i I introduced that argument in this commit 0279b9d but to be honest I don't remember why.
I will vote to remove the Strict argument not sure if is that useful

[GoldenChrysus]

The change in position_list.py causes an out of range exception as len(self.analysed_legals) >= move_number does not mean that self.analysed_legals contains move_number. (e.g. if self.analysed_legals has a length of 1 and move_number is 1, that does not mean self.analysed_legals contains index 1; it only contains 0).