mpolyList arithmetic with single mpoly

``` r
library("mpoly")

mp(c("x", "y")) + mpolyList(mp("2"))
#> Error: e1 and e2 must have equal length.

mp(c("x", "y")) + 2
#> Error: e1 and e2 must have equal length.

mp(c("x", "y")) + mp("2")
#> Warning: Incompatible methods ("+.mpolyList", "+.mpoly") for "+"
#> Error in mp(c("x", "y")) + mp("2"): non-numeric argument to binary operator
```

^{Created on 2019-03-20 by the [reprex package](https://reprex.tidyverse.org) (v0.2.1)}