Not able to chart a field with partly non-numerical type(s)

When a field is of a numeric type in parts of the indices of the current timerange and of non-numeric type in the rest, a compound type is generated and several actions are disabled, e.g. charting the field/generating a line chart of the field's values over time.

This might be improvable, e.g. by:

* In case of an operation being performed on a field with mixed numeric/non-numeric types in the set of indices, filter out indices where the field is non-numeric and perform the operation on the remaining ones
* results in gaps in e.g. the resulting chart
* try to convert values on the fly, e.g. by employing painless

Refs #6317.