PeriodPartition breaks EMGUI

[Zetison]

PeriodPartition, introduced in TimeStruct 0.9.12, creates a bug when used together with EMGUI. Running the GUI on the introduced tests of this PR illustrates this with the error

Type TimeStruct.OperationalPeriod{TimeStruct.SimplePeriod{Float64}} can not be used as index for a PartitionProfile{Float64}

Ideally, a solution to this should be first implemented in EMGUI before this PR can be merged.

[JulStraus]

The problem is within the processing of EnergyModelsGUI which does not fully support the new time profile types and subdivisions. The reason for that is that partition_duation can be different for different type of nodes. As a consequence, it cannot be directly incorporated within the TimePeriod as it is the case for all other potential time structures. Subsequently, a PartitionProfile cannot be indexed by a TimePeriod as the information of the partition is not accessible as this level, despite being theoretically able to use them as indices.

That being said, it requires a rework of the plotting functionality to work properly for all nodes that have variables or parameters that require PeriodPartition. As I do not consider it in this case prudent to wait for the implementation in EMGUI (which may take a bit of work), I say that we instead still merge PR 12 in EnergyModelsFlex and provide a warning in the README.md and the documentation instead.