Feature request snapshot filter

### System
**qcodes branch**
feature/dataset

**feature**
The snapshot object contains all the parameters present in all devices in the setup. I guess for most setups that would mean hundreds of different parameters. Of these, we generally have special interest into some. I propose to add an extra property to the DataContainer, i.e. MetaFilters. This property has the following form:
{'filtername_abc':
['path_to_a_meta_data_end_point', ['path_to_b_meta_data_end_point', ['path_to_c_meta_data_end_point'],
'filtername_def':
['path_to_d_meta_data_end_point', ['path_to_e_meta_data_end_point', ['path_to_f_meta_data_end_point']}
The metadata filter can be used to identify the important parameters active in a setup. For example, we have voltage source with 50 channels, but for the presently loaded sample, we only use a subset of channels. In the filter we can identify the used channels so, later on, we can easily ask for only those metadata endpoints.

Start by locating the DataContainer and snapshot implementation; no source files, tests, or entry points are named in the issue. Clarify the proposed MetaFilters structure and expected lookup behavior, then define completion as a reviewed filter API that can identify selected metadata endpoints for a setup.

Written by the indexing model from the issue text.