Add pagination and sorting mechanism

[sosna]

Fix #50
Fix #151

[dosse]

@sosna I made the following changes:

Rest-cheat-sheet:

• syntax corrections for e.g.

Pagination:

• added series key for serieskeyonly queries

Sort:

• by default, always group observations into time series and sort by ascending time period
• within components sort by code ID or non-coded value

Something that still needs clarification is the following:

If the data are sorted by the components in the order stated in the sort clause, then sort=TIME_PERIOD:desc would do the following: It would first sort in descending order of TIME_PERIOD and then by the other components in a nondeterministic way, e.g.,

  FREQ,DIM1,TIME_PERIOD,OBS_VALUE
  A,A,2022,1.0654
  A,B,2022,1.0261
  A,A,2021,1.0752
  A,B,2021,1.0730

In consequence, the observations will not be grouped by time series. I don't think that this was intended with sort=TIME_PERIOD:desc, right?

This could be solved by saying:
The data are sorted first in a nondeterministic way for any component not included in the sort parameter and then in the order of components in the sort parameter. In this way, sort=TIME_PERIOD:asc would also be the default sorting value if the data has a time dimension.

Ok for you?