add the MISCIBLE keyword

it's only specified by R300, but it is required TLMIXPARAM which is used by
opm-polymer...
fix the PLYMAX keyword

- fix typo: use "POLYMER" instead of "PLOYMER"
- according to the Eclipse RM, the size of the table is given by the
  "NTMISC" item of the "MISCIBLE" keyword...
add the TLMIXPAR keyword

this is used by opm-polymer...