add Lossless line and converter

docs/src/formulation_library/DCModels.md

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+# DC Models formulations
+
+!!! note
+    Multi-terminal DC models are still in early stages of development and future versions will add a more comprehensive list of formulations
+
+---
+
+## LossLessLine
+
+`LossLessLine` models are used with `PSY.DCBranch` models.
+
+```@docs
+LossLessLine
+```
+
+**Variables:**
+
+- [`FlowActivePowerVariable`](@ref):
+  - Bounds: ``(R^\text{min},R^\text{max})``
+  - Symbol: ``f``
+
+**Static Parameters**
+
+- ``R^\text{from,min}`` = `PowerSystems.get_active_power_limits_from(branch).min`
+- ``R^\text{from,max}`` = `PowerSystems.get_active_power_limits_from(branch).max`
+- ``R^\text{to,min}`` = `PowerSystems.get_active_power_limits_to(branch).min`
+- ``R^\text{to,max}`` = `PowerSystems.get_active_power_limits_to(branch).max`
+
+Then, the minimum and maximum are computed as `R^\text{min} = \min(R^\text{from,min}, R^\text{to,min})` and `R^\text{max} = \min(R^\text{from,max}, R^\text{to,max})`
+
+**Objective:**
+
+No cost is added to the objective function.
+
+**Expressions:**
+
+The variable `FlowActivePowerVariable` ``f`` is added to the nodal balance expression `ActivePowerBalance` for DC Buses, by adding the flow ``f`` in the receiving DC bus and subtracting it from the sending DC bus.
+
+**Constraints:**
+
+No constraints are added to the function.
+
+---
+
+## LossLessConverter
+
+Converters are used to interface the AC Buses with DC Buses.
+
+```@docs 
+LossLessConverter
+```
+
+**Variables:**
+
+- [`ActivePowerVariable`](@ref):
+  - Bounds: ``(P^\text{min},P^\text{max})``
+  - Symbol: ``p``
+
+
+**Static Parameters:**
+
+- ``P^\text{min}`` = `PowerSystems.get_active_power_limits(device).min`
+- ``P^\text{max}`` = `PowerSystems.get_active_power_limits(device).max`
+
+
+**Objective:**
+
+No cost is added to the objective function.
+
+**Expressions:**
+
+The variable `ActivePowerVariable` ``p`` is added positive to the AC balance expression `ActivePowerBalance` for AC Buses, and added negative to `ActivePowerBalance` for DC Buses, balancing both sides.
+
+**Constraints:**
+
+No constraints are added to the function.