support mixed fom/rom: default lspg and default implicit galerkin

[fnrizzi]

default lspg implicit

• [x] implementation
• basic tests
  • [x] rom-fom-rom (bdf1)
  • [x] rom-fom-rom (bdf2)
  • [x] fom-rom-fom (bdf1)
  • [x] fom-rom-fom (bdf2)
• more complex tests
  • [x] fom-fom-rom-rom-fom (bdf2)

default galerkin implicit

• [x] implementation
• basic tests
  • [x] rom-fom-rom (bdf1)
  • [x] fom-rom-fom (bdf1)
  • [ ] rom-fom-rom (bdf2)
  • [ ] fom-rom-fom (bdf2)

[fnrizzi]

this needs to be rebased and also reverified because even if it works, maybe is could be improved or simplified