Remove methods getCovariatesNames, getIOVNames, etc.

[luyckxn]

These methods should be reworked as follows:

• getCovariateNames to getCovariates()
• getIOVNames to getIOVs
• getOccasionNames to getOccasions()
• getTimeVaryingCovariateNames to getTimeVaryingCovariates()
• getEventCovariateNames to getEventCovariates()

Then, calling the generic method getNames() should return a vector of names.