Check if the file exists

[timfelle]

Ensure calls to file readers actually check if the file they are supposed to read exists.

[timofeymukha]

Would it be better to add a check_file_exists TBP to generic_file_t with

    inquire(file=this%fname, exist=file_exists)
    if (.not. file_exists) then
       call neko_error('File does not exist: '//trim(this%fname))
    end if

and only have call check_file_exists() in the subtypes?