Refactor existing functionality to make intake-esm robust and extensible

[andersy005]

Notable Changes

• Partially addresses Needed Code Refactoring  #70 by cleaning up test directory.

• Closes example_input is out of date #73

• Building CMIP5 and CMIP6 doesn't rely on directory structure anymore. Just like CESM data holdings, attributes are extracted from the filename, and any additional attributes (e.g. grid_label, version) are extracted from the file fullpath.

• All collections inherit from Collection base class located in intake_esm/collection.py.

• The structure of the input YAML file is now consistent across all collections.

Example snippets:

• CMIP6

name: cmip6_test_collection
collection_type: cmip6
data_sources:
 BCC-CSM2-MR:
   locations:
    -  name: SAMPLE-DATA
       loc_type: posix
       direct_access: True
       urlpath: ./tests/sample_data/cmip/CMIP6/CMIP/BCC/BCC-CSM2-MR
       exclude_dirs: ['*/files/*', 'latest']
       file_extension: .nc

   extra_attributes:
     mip_era: CMIP6
     activity_id: CMIP
     institution_id: BCC

 BCC-ESM1:
   locations:
    -  name: SAMPLE-DATA
       loc_type: posix
       direct_access: True
       urlpath: ./tests/sample_data/cmip/CMIP6/CMIP/BCC/BCC-ESM1
       exclude_dirs: ['*/files/*', 'latest']
       file_extension: .nc

   extra_attributes:
     mip_era: CMIP6
     activity_id: CMIP
     institution_id: BCC

• CESM

name: cesm1-le
collection_type: cesm
data_sources:
  CTRL:
    locations:
      - name: SAMPLE-DATA
        loc_type: posix
        direct_access: True
        urlpath: ./tests/sample_data/cesm-le
        file_extension: .nc
    extra_attributes:
      component_attrs:
        ocn:
          grid: POP_gx1v6
      case_members:
        - case: b.e11.B1850C5CN.f09_g16.005
          sequence_order: 0
          ensemble: 5
          has_ocean_bgc: True
          year_offset: 1448
  20C:
    locations:
      - name: SAMPLE-DATA
        loc_type: posix
        direct_access: True
        urlpath: ./tests/sample_data/cesm-le
        file_extension: .nc
    extra_attributes:
      component_attrs:
        ocn:
          grid: POP_gx1v6
      case_members:
        - case: b.e11.B20TRC5CNBDRD.f09_g16.001
          sequence_order: 0
          ensemble: 1
          has_ocean_bgc: True

@matt-long, your review/comments would be helpful prior merging this into master.