Option to load UNSMRY data as 32 bit floats

According to the Eclipse documentation, the vector parameter values in UNSMRY are stored with the data type `REAL`, which is described as single precision (4 byte) floating point reals.

In `ecl`, at least from the `python` side (think also `C++` side, at least what I've seen), all values are returned as `float64`, giving a substantial memory overhead. For a single simulation, it doesn't have much to say, but if you aggregate to ensembles, the difference is significant.

Would be nice to be able to get `float32` directly (as an optional flag to avoid when requesting e.g. numpy arrays or pandas DataFrames (preferably also on the `C++` side to limit memory usage also on that side, though the `python` output of the bindings are the most important for my personal use).