have functions to read other properties from files

[lbluque]

Similar to #30, at ALS8.3.2 we need to be able to read other properties from h5 files.
Originally I had an extra property that was read when calling the function read_als_832h5, but in #25 based on #20 I removed this to keep the API the same across other readers.
I am thinking that having a metadata reader module where functions from different facilities return a dictionary with metadata that is deemed important is a good way to go.