added support for lazy loading DPC and FFT

[din14970]

Description of the change

in #2564 and #2567 I added support for loading DPC and FFT signals from the FEI .emd format but didn't know how to support lazy reading. It's a tiny gap but after learning a bit about Dask this should now be resolved with this PR.

Progress of the PR

• Change implemented (can be split into several points),
• update docstring (if appropriate),
• update user guide (if appropriate),
• add entry to CHANGES.rst (if appropriate),
• add tests,
• ready for review.

[codecov]

Codecov Report

Merging #2651 (42af7bd) into RELEASE_next_patch (33dc8de) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@                 Coverage Diff                 @@
##           RELEASE_next_patch    #2651   +/-   ##
===================================================
  Coverage               76.41%   76.42%           
===================================================
  Files                     202      202           
  Lines                   29678    29680    +2     
  Branches                 6490     6490           
===================================================
+ Hits                    22679    22683    +4     
+ Misses                   5228     5227    -1     
+ Partials                 1771     1770    -1     

Impacted Files	Coverage Δ
hyperspy/io_plugins/emd.py	66.63% <100.00%> (+0.29%)	⬆️

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[din14970]

Not sure why it is failing on py3.6

[ericpre]

https://docs.dask.org/en/latest/changelog.html#id40

It most likely need dask > 2.3.0