Create Subject and Sample Classes for operating them more easier and readable

[LinkunGao]

• [x] Change all field_name to element
• [ ] Base useage structure for these Subject and Sample classes:

  subjects = []
  for subject_id in [1, 2, 3]:
      samples = []
      for sample_id in [1, 2]:
          sample = sm.Sample()
          sample.add(
              "../resources/example_dataset/Breast_MRI_00{0}/sequence{1}/".format(subject_id, sample_id),
              metadata={
                  "sample experimental group": "experimental",
                  "sample type": "DCE-MRI Contrast Image {0}".format(sample_id),
                  "sample anatomical location": "breast tissue"})
          samples.append(sample)
      subject = sm.Subject()
      subject.add(
          samples,
          metadata={
              "subject experimental group": "experimental",
              "sex": "Female",
              "species": "human",
              "strain": "tissue",
              "age category": "middle adulthood"})
  dataset.add_subjects(subjects)

  subject = dataset.get_subject(1)
  subject.add_values(
      element='age',
      value=30, append=False)

  sample = subject.get_sample(2)
  sample.add_values(
      element='some_element',
      value=30, append=False)