[ENH] Start steel_bridge module in ec3 with partial safety values.

[domthom21]

Is your feature request related to a problem? Please describe. Currently, we are missing Eurocode 3-2 "Steel bridges"

Describe the solution you'd like I suggest adding a module eurocodedesign.standard.ec3.steel_bridges

The __init__.py will contain the partial safety factors, similar to __init__.py of ec3

Add a context manager to ec3, which allows setting the context "EC3Context.steel_bridges". The normal context will be "EC3Context.None" Context itself is an enum, which will be extended later for silos, towers, etc.

Add gamma_M0(), gamma_M1()... similar to ec3.__init__.py which can support national annex like ec3

Add to ec3.__init__.py that the gamma functions load the default values or forward the call to the corresponding context functions, like the steel_bridge gamma functions.

Add context as an additional optional argument to the gamma functions in ec3.

The context functionality will have the same functions like the national annex core module.

Describe alternatives you've considered Add functionality directly in existing functions, however I think seperating the functionality and forwarding calls is preferable

Additional context

• Should the enum value names be uppercase?
• Maybe the first enum value should be buildings instead of none
• We should keep in mind, how we will support that only some materials are valid in the steel_bridge context