Refactoring of class "grid"

1. Improved naming
2. Added “this” in front of class fields
3. Most common case moved first in if statements and changed to positive
conditions
4. Added one more case in “RevealCell” method for visited cells
5. New naming of adjacent cells in “NeighbourMinesCount” method
6. Renamed “putall” method to” InitializeMineField”
7. Extracted two methods from” InitializeMineField“  for generation of
random mine position (“GenerateRandomPositions” method) and mine
placement on the field (“FillMines” method)
8. Removed unused method “put”
9. Method “reset” lexicographically close to “Reset” replaced with
“ResetGridCells”
10. Removed  unused property “RevealedCount”
11. Renamed method “mark” to “MarkAndRevealEmptyFields”
12. Removed unused method “get”
13. Extracted methods “GenerateHorizontalFieldBorder”,
“GenerateColumnsNumbers” and “GenerateRows” from ToString() override,
reducing repeating code and improving readability.
14. Chosen appropriate naming  for loop control variables