Add helper methods to verify values

[schloerke]

Something similar to

checkmate::expect_count
checkmate::expect_integer
checkmate::expect_number
checkmate::expect_numeric
checkmate::expect_double

checkmate::expect_data_frame
checkmate::expect_tibble

checkmate::expect_list
checkmate::expect_names


checkmate::expect_character
checkmate::expect_string
checkmate::expect_factor
checkmate::expect_logical
checkmate::expect_int
checkmate::expect_class
checkmate::expect_atomic
checkmate::expect_scalar

checkmate::expect_function

checkmate::expect_set_equal

... but with gradethis friendly error messages.

Can use the checkmate:test_*() methods to help determine if valid