VisList properties overidden upon refresh_source initialization

[dorisjlee]

When properties are specified in the Vis and fed inside the VisList, the refresh_source goes through the parser, validator and compiler. At the compiler stage, the Vis properties gets overridden by the automatically determined values. We should tolerate specified title, mark and other Vis properties to override the automatically determined values.

This is an example test that evaluates this, the last two line fails because the title and mark takes on the new values.
This example should be added to the test suite once we resolve this issue.

df = pd.read_csv("lux/data/olympic.csv")
df["Year"] = pd.to_datetime(df["Year"], format='%Y') # change pandas dtype for the column "Year" to datetype
from lux.vis.VisList import VisList
from lux.vis.Vis import Vis
vcLst = []
for attribute in ['Sport','Year','Height','HostRegion','SportType']: 
    vis = Vis([lux.Clause("Weight"), lux.Clause(attribute)],title="overriding dummy title",mark="line")
    vcLst.append(vis)
vc = VisList(vcLst,df)
for v in vc: 
    assert v.title=="overriding dummy title" # AssertionError
    assert v.mark=="line" # AssertionError

[dorisjlee]

This issue also causes the string title property for Date to be overridden.

Fixing this issue should also resolve the Date issue.

[dorisjlee]

Now supporting a custom title override by avoiding unnecessary calls to enumerate_collection. Mark and channel override non-trivial since we need to extend determine_encoding channel assignment to work with the non-default, custom mark.

[dorisjlee]

Currently, we don't support mark override on Vis, since there can be conflicts with the input intent (e.g. user intent involves only a single quantitative attribute, but sets mark as scatterplot). We can revisit this once there are a better validator and parser that catches these issues. For now, _mark is set as private property.