Change naming convention

[ClaraHapp]

To be in line with other packages for functional data (fda, refund), renaming of the slots representing a functional data object is suggested:

• the slot representing observation points (xVals) should be renamed to "argvals"
• the observed functional data (X) should be changed to "x"

[ClaraHapp]

slot for observation points is now renamed to argvals observed functional data is kept as "X"