Find proxyfmu using parent executable

[markaren]

Closes #40

Unsure if this works on linux without linker flags.

Also resolves #39

[ljamt]

Works well on windows. Unfortunately not on linux.
When running cosim-cli from a path different from where the executable is located is fails:

[proxyfmu] Booting FMU instance 'mass'..
terminate called after throwing an instance of 'boost::process::process_error'
  what():  execve failed: No such file or directory
Aborted

[markaren]

Then it might be the rdynamic linker option that we need, but the error suggests that it actually works, but it might not like the string format of the executable path.

[ljamt]

BTW, works well on linux if cosim-cli is called from its path.

[markaren]

Should work now. Also fixed an issue where proxyfmu would only work in a bash shell in windows. It never used the .exe postfix...