add checks for S7 class

[StefanoMezzini]

the package currently does not check for S7 classes. Since most tests are based off of !is.S4(), S7 classes are treated as S3.

[StefanoMezzini]

We should wait for adding R 4.2.0 to Requires so that we can check inherits(obj, <S7 class>) because inherits() only works with S7 starting form R >= 4.2.0.

[joethorley]

testing code will be something like

 S7_generator <- S7::new_class("exampleS7class")
  expect_true(inherits(S7_generator(), S7_generator))
  expect_false(vld_s3_class(S7_generator(), "S7ClassGenerator"))