Initialise alt with type specific 0 instead of Float64

[lwabeke]

Without this change constructors using Float32 variables fail.

[lwabeke]

The keyword constructor is still a problem:
LLA(;lat=NaN,lon=NaN,alt=0.0)
But not sure how to fix this.

[coveralls]

Coverage decreased (-1.09%) to 77.984% when pulling e6276a1 on lwabeke:bugfix-on-constructor into 378259b on JuliaGeo:master.

[lwabeke]

Quick attempt to try to address
#29

[andyferris]

Thanks @lwabeke - looks good.

Keyword constructors are currently not type-stable (meaning, in general in Julia, unless they have a fully-defined type like alt::Float64 = 0.0, but this of course doesn't fix your problem) so I'm not worried about this. Have you tried alt = zero(lon), as default values can usually depend on earlier ones (but I'm not as certain about keyword arguments).

[lwabeke]

The problem in that keyword constructor is that NaN if already Float64, thus unless all 3 are specified as being a different type, it will cause problems. But I'm also not too worried about that case.

[c42f]

Eek, we should have merged this ages ago, thanks for the patch.