Projection of other observation

[jiangyi15]

Support projection of other observation.

plot:
    index:
          a/b/c: {}  # for  data["a"]["b"]["c"] 
    extra_vars:
          data:
               expr:  m_R_BC + m_R_BD
               where:
                      m_R_BC: ["mass", "R_BC"]

[jiangyi15]

support additional observation from function

idx = config.get_data_index("mass", "R_BC")
def f(x):
    return data_index(x, idx)

config.plot_partial_wave(extra_plots=[{"name": "function", "readdata": f}])