get_sections_by_value, get_blocks_by_value

Id like to suggest methods which receive a value and return all sections/blocks/data_arrays that contain that value. Something like:

``` python
def get_sections_by_value(value, nix_file):
return nix_file.find_sections(lambda x:any([any([value in i for i in
e.values]) for e in
x._properties]))

def get_blocks_by_value(value, nix_file):
return filter(lambda x:len(get_sections_by_value(value, x.metadata))>0,
nix_file.blocks)

```

but with all the corner cases and type issues solved and of course, well not in python. I think "give me all recordings in which i wrote something like whatever" is quite a usual use case. (Yes i know that this is most probaly non trivial)