merge GA version of modem related code with the geophysics version (close #9)

[YingzhiGou]

to do

• [x] move objects in modem.py into their own files

• [x] pass all modem.py related tests

• [x] pass all GA modem tests

• [x] merge Data object

  • [x] merge changes
  • [x] merge tests of the modem.py and GA tests
  • [x] pass all merged tests
  • [x] remove old tests

• [x] merge Model object

  • [x] merge changes
  • [x] merge tests of the modem.py and GA tests
  • [x] pass all merged tests
  • [x] remove old tests

• [x] merge Covariance

  • [x] create tests for both modem.py and GA version
  • [x] merge changes
  • [x] merge tests of the modem.py and GA tests
  • [x] pass all merged tests
  • [x] remove old tests - [ ] merge Residual

• [x] compare changes in modem_new.py_AK2017Feb10

• [x] update documentation

• [x] cleanup

  • [x] remove GA version
  • [x] remove modem_new.py_AK2017Feb10
  • [x] create new modem.py or restructure modem module