Display all images in scaled voxels

See merge request fsl/fsleyes/fsleyes!286

comments about origin being centre, not corner, of voxel (0,0,0).

Bf/timeseriesprofile

See merge request fsl/fsleyes/fsleyes!283

TEST: Update update test - hadn't realised that there was a unit test for it

See merge request fsl/fsleyes/fsleyes!281

Update update check

See merge request fsl/fsleyes/fsleyes!279

ENH,BF: Mesh interpolation, allow the same cmap/lut file to be specified more than once on command line

Closes #196 and #197

See merge request fsl/fsleyes/fsleyes!278

options - I'm fairly sure this is unnecessary cruft

a LUT. Fix interp widget enabled state

MeshOpts

MeshOpts.interpolation. Implement interpolation on 2D canvases (when colouring
2D cross section outline with data), in addition to 3D

searched for by file name or by key. Used by parseargs so that it does not
attempt to re-register luts/cmaps specified on command line

MNT: clarify step in docker image script [skip-ci]

See merge request fsl/fsleyes/fsleyes!277

BF: Add display listeners after image texture is created.

See merge request fsl/fsleyes/fsleyes!275

contexts (i.e. fsleyes render), GLVolume.refreshImageTextures could result in
an infinite loop - image data access causes ImageWrapper to notify listeners,
calling GLVolume.__displayRangeChanged -> GLVolume.updateShaderState, which
uses idleWhen to call a function when the image texture is ready, which it
never will be.

TEST: fix melodic power spectrum test

See merge request fsl/fsleyes/fsleyes!273