Overly-strict validation for `TimeSeries.legendCalcs`

[JoshKarpel]

The TimeSeries.legendCalcs field validates the values that can go in its list:

grafanalib/grafanalib/core.py

Lines 2315 to 2342 in 3ed1a9c

	legendCalcs = attr.ib(
	factory=list,
	validator=attr.validators.deep_iterable(
	member_validator=in_([
	'lastNotNull',
	'min',
	'mean',
	'max',
	'last',
	'firstNotNull',
	'first',
	'sum',
	'count',
	'range',
	'delta',
	'step',
	'diff',
	'logmin',
	'allIsZero',
	'allIsNull',
	'changeCount',
	'distinctCount',
	'diffperc',
	'allValues'
	]),
	iterable_validator=instance_of(list),
	),
	)

Other chart types don't, e.g. BarChart:

grafanalib/grafanalib/core.py

Line 4494 in 3ed1a9c

legendCalcs = attr.ib(factory=list, validator=instance_of(list))

At least on my Grafana instance, there are more legend calculations available, such as p95 (not sure if this a plugin or just some new feature that's been added since the code was originally written). It would be convenient if this client-side validation was dropped, in case the user's Grafana has more options available.