Easy definition of sanitized substitutions

[jlstevens]

This PR aims to address issue #264.

[jlstevens]

I think this PR is ready to merge. Here is an example of what it allows you to do:

layout = (hv.Image(np.random.rand(10,10), 
         group=('Spectrum', 'Frequency spectrum'), 
         label=('Glucose', '$C_6H_{12}O_6$'))
+ hv.Image(np.random.rand(10,10), 
         group=('Spectrum', 'Frequency spectrum'), 
         label=('Water', '$H_2O$')))

Now you can select the two parts of the layout conveniently with layout.Spectrum.Glucose or layout.Spectrum.Water while showing the chemical compound names in the title (before you had to use some awkward, auto-generated sanitized string).

To make this less awkward, you can do this using the new Aliases helper function:

al = hv.util.Aliases(Spectrum='Frequency spectrum', 
                     Water='$H_2O$', 
                     Glucose='$C_6H_{12}O_6$')

(hv.Image(np.random.rand(10,10), group=al.Spectrum,  label=al.Glucose)
+ hv.Image(np.random.rand(10,10), group=al.Spectrum, label=al.Water))

Lastly, the same principles apply setting dimension names using tuples:

im =hv.Image(np.random.rand(10,10), 
             kdims=[('Lambda', '$\Lambda$'), ('Joules', 'Energy ($J$)')])

Which now makes methods such as select easier:

im.select(Lambda=(-0.2, 0.2), Joules=(-0.3, 0.3))

Of course you can use the Aliases helper here too! You can define as many of these Aliases as you desire if you wish to have 'namespaces' keeping your aliases separate (i.e one for each of groups, labels and dimension names).

After I add some tests, I think this is ready to merge and I definitely think it will help scientists (and anyone using LaTeX and special symbols really!) use HoloViews productively.

[jlstevens]

I've now added unit tests (based on the examples above) and once they pass, I think it is ready to merge (unless you have any comments!).

It is worth mentioning one limitation of this new system, the group/label/dimension name stays a string (the long string, not the short alias) which means your alias definitions won't be pickled together with the object.

Aliases are defined as dictionaries (the aliases parameter) on the three global functions: hv.util.label_sanitizer, hv.util.group_sanitizer, hv.util.dimension_sanitizer. Restoring your aliases comes down to restoring these dictionaries on these three objects appropriately.

[philippjfr]

Looks good, very happy to finally have a nice way to specify aliases. Since the tests are passing and I have no specific comments I'll go ahead and merge.

[jbednar]

Excellent! Looks great.