Adapt read_vpts()and vpts object to VPTS CSV format

[peterdesmet]

Current functionality

Read a time series of vertical profiles (vpts) from file

read_vpts(
  file,
  radar,
  lat,
  lon,
  height,
  wavelength = "C"
)

Reason for change

• VPTS CSV is now a standardized format for VPTS data. We need a function to read that data
• It is intended to replace the "A text file containing the standard output (stdout) generated by vol2bird (or the package function calculate_vp)."

Questions

• Should data be converted to VPTS object or remain a data frame? This depends on whether we want to keep vpts objects around or make those regular data frames. @adokter
• VPTS objects have some conventions regarding column order and names. I think those should be adopted in VPTS (data frames).
• Should it be possible to read from a URL in addition to single file path (supported by readr, see read_vpts(): support reading remote files #420)
• Should we still support the legacy stdout objects? That requires keeping around all the parameters too.

Todo

• Remove radar parameter. It is included in data as radar
• Remove lat parameter. It is included in data as radar_latitude
• Remove long parameter. It is included in data as radar_longitude
• Remove height parameter. It is included in data as radar_height
• Remove wavelength parameter. It is included in data as radar_wavelength
• Allow to read a list of file paths, rather than single file. Those files will be combined.
• Add readr as dependency
• Distinguish between NA and NaN (see data read with read_stdout does not distinguish between NA and NaN (nodata and undetect) #40)