New function: validate metadate_file

[LinkunGao]

Description:

• validate metadata_file and field_name: ignore their case, space and underscores
  • useage:

      dataset.list_elements(metadata_file="dataset_description", version="2.0.0")
      dataset.list_elements(metadata_file="datasetdescription", version="2.0.0")
      dataset.list_elements(metadata_file="dataset description", version="2.0.0")
      dataset.get_metadata(metadata_file="dAtasetDescription")
      des_schema = schema.get_schema("dataset Description")
      des_schema.get('subtitle')
      des_schema.get('sub title')
      des_schema.get('Subtitle ')

Related issue(s):

106

Test Environment:

Specify the testing enviroment. e.g. OS, Python version