fixed: workaround issues with ExternalProject_Add

if you use the builtin git update command, a build is triggered
every time. instead, manually use a checkout
Merge pull request #18 from akva2/eigen3_fix_continuous_installs

fixed: workaround issues with ExternalProject_Add
fixed: autodiff depends on opm-core
Merge pull request #19 from akva2/opm_depends

fixed: autodiff depends on opm-core