TrashInterface does not validate cell types

[budnyric]

This can lead to undefined behavior. From TrashInterface:
symbol_grid[y][x] = type_options[ grid.At(x,y) ].symbol;
The use of brackets here allows reading beyond the end of the array when illegal cell type ids are specified.
Suggestion: Change use of brackets to .at() to instead throw an exception if this occurs.

[amantham20]

adding on this, users can place an agent even on top of walls. Might need to throw an exception when initial starting point is on top of a wall