Relax AbstractFloat to Real

[aplavin]

There are quite some Julia packages that provide numbers <: Real but not <: AbstractFloat. Currently, one cannot use them with SkyCoords:

julia> using SkyCoords, MonteCarloMeasurements

julia> ICRSCoords(1±0.1, 0±0.1)
StackOverflow

Can we relax the constraints in SkyCoords to <: Real? I'll be happy to make a PR for that.

[cgarling]

I am not very familiar with this package but I would guess that AbstractFloat is necessary because of the mod2pi call in the constructor, which given (for example) an integer argument would return a floating point value, therefore changing the type of the first argument.

SkyCoords tries to convert arguments to a common floating point type when constructed from generic reals, but if float() style conversion is not implemented for the alternate type this will not help.

[aplavin]

Number types like MonteCarloMeasurements support all kinds of math (+/mod/...). They just subtype Real and not AbstractFloat, while the latter is currently required by SkyCoords.

[aplavin]

@mileslucas @giordano wdyt?