Do not use using namespace mv in headers

[alxvth]

We should not write using namespace mv header files that others might consume since then they would unknowingly also use that namespace constantly.
This also follows the c++ core guideline Don’t write using namespace at global scope in a header file:

Reason: Doing so takes away an #includer’s ability to effectively disambiguate and to use alternatives. It also makes #included headers order-dependent as they might have different meaning when included in different orders

We have done this already across the core's public API, but the core and data plugins have not been as compliant.

This PR might break the build of some plugins, which will have to add using namespace mv back to their source.

I've tested:

• BinIO
• ClusterIO
• ClusteringPlugin, Add mv namespace ClusteringPlugin#3,
• ClusterMeansPlugin, Add mv namespace ClusterMeansPlugin#5
• ClustersFromPointsPlugin, Add mv namespace ClustersFromPointsPlugin#20, Add mv namespace (part 2) ClustersFromPointsPlugin#22
• CsvLoader
• CsvWriter
• DE-MultiSpecies
• DifferentialExpressionPlugin
• DimensionsViewerPlugin, Add mv namespace DimensionsViewerPlugin#27
• ENVILoader
• ExamplePlugins
• ExtCsvLoader
• HDF5Loader, Add mv namespace HDF5Loader#39
• HeatMap
• ImageLoaderPlugin, Add mv namespace ImageLoaderPlugin#129
• ImageTransformationPlugin, https://github.com/ManiVaultStudio/ImageTransformationPlugin/pull/7
• ImageViewerPlugin, Add mv namespace ImageViewerPlugin#153
• JupyterPlugin, Add mv namespace JupyterPlugin#63
• MeanShiftClustering
• ParallelCoordinatesPlugin
• PcaPlugin, Add mv namespace PcaPlugin#39
• PointDataConversionPlugin
• RefinePlugin, Add mv namespace RefinePlugin#13
• Scatterplot
• SparseH5DataAccessPlugin, Add mv namespace SparseH5DataAccessPlugin#19
• SpectralViewPlugin, Add mv namespace SpectralViewPlugin#18
• SunburstClusterPlugin, Add mv namespace SunburstClusterPlugin#20
• UMAP-Plugin, Add mv namespace UMAP-Plugin#31
• t-SNE-Analysis, Add mv namespace t-SNE-Analysis#166
• VolumeProjectorPlugin, Add mv namespace VolumeProjectorPlugin#10

All linked PRs in plugins do not depend on this PR (they are compatible with the current core), but are required after this PR is merged.

If there is no linked PR, the plugin does not need to be updated.