tidied error outputs

[asyates]

Stopping errors being output during processing of new codes (e.g. stack2, mwcs2, dtt2, dvv) when msnoise is functioning as intended, as makes it confusing (gives impression not working)

Largely, just stopped using params.all_components in the loops as this throws an error when files don't exist (e.g. for pairs and components combinations that were never intended to be processed). Correction in api.py, compute_dvv function a bit messy, prob cleaner way to do it.

Also was an error being output in api.py, xr_save_dvv function when only one pair for given components (with dataframe.stack().stack() then throwing an error). Now fixed.

Fixes Issue #320

[asyates]

Also re-added fix for previous issue #252 (same code as previous), which solves problem of df * mwcs_step being a non-integer (previous pull request with more description at #253).

[ThomasLecocq]

looks good to me, ready to merge if you're happy with it ?

[asyates]

yes sir