Checks for unknown dataspace & unknown dimension in Model

[tanner-andrulis]

Multiple students in class have had "what() map::at" errors because they misspell a dimension name or dataspace name. The mapper checks for this and gives a descriptive error (e.g., a message saying that Timeloop couldn't parse a "keep" list because it doesn't recognize "Ouptus" as a dimension). We should have these checks in the model as well.