[doc] Generic EWS workflow

[alexgenin]

Make sure the generic ews workflow is documented the following functions should have their respective roxygen comments:

• [x] generic_spews (main function)
  • [x] plot methods
• [x] indictest.genericspews* (assess significance)
  • [x] plot methods

[alexgenin]

Good for now.

[alexgenin]

• [ ] Make sure it is clear that coarse-graining is necesary

[alexgenin]

• [ ] Explain the output value (e.g. z-score)

[alexgenin]

Done.