PhysicalConstants cannot be instantiated inside of a Dict

[neil-glikin]

Example:
Dict("D5/2" => PhysicalConstant(4.1115503183857306e14, "Hz"))
Returns the error "TypeError: in new, expected PhysicalConstant, got Float64". This seems to be caused by the Base.convert for PhysicalConstants added in constants.jl. I suspect there's something not quite right with this usage of convert, as defining this method alone breaks all instantiation of PhysicalConstants (even outside of Dicts); the Base.Fix2 method seems to patch this up, but does not fix the problem when the instantiation takes place inside a Dict. I do see that some sort of Base.convert method is necessary, as it allows mathematical operations with regular numbers, but not sure how it would need to be fixed.

It would be nice to be able to do this so that in species files e.g. /src/species/ca40.jl we can directly define energies and transition matrix elements as PhysicalConstants within dictionaries.

[marwahaha]

@neil-glikin - is this still an issue?

[neil-glikin]

@marwahaha Yes, I created a new more general issue #53