add possibility to collapse during read.jdx

[cbeleites]

Currently, read.jdx offers to collapse multiple spectra inside one JCAMP-DX file. For large files (such as "fileio/jcamp-dx/GMD_20111121_MDN35_ALK_JCAMP-shortened.txt= 'fd2e686f5dc78691c22033805ed56463'.txt", the intermediate list may be too memory consuming to read.

Solutions:

• [ ] offer collapsing already after a few spectra have been read.
  • [ ] implement
  • [ ] unit test
  • [ ] document
  • [ ] vignette
• [ ] offer reading only a subset of spectra
  • [ ] implement
  • [ ] unit test
  • [ ] document
  • [ ] vignette

[cbeleites]

Difficulty was not read.jdx but rather inefficient collapse => fixed in commit bd0f71b