Give a meaningful error message for convert(SVector,::Vector) or SVector(::Vector)

[dlfivefifty]

In the FAQ it is explained why conversion of a Vector to an SVector is not allowed. While I'm not sure I agree with this, if the current behaviour is kept then the error message should be explanatory: instead of

julia> convert(SVector,[1,2,3])
ERROR: MethodError: Cannot `convert` an object of type Type{StaticArrays.SArray{Tuple{S},T,1,S} where T where S} to an object of type StaticArrays.Size
This may have arisen from a call to the constructor StaticArrays.Size(...),
since type constructors fall back to convert methods.
Stacktrace:
 [1] convert(::Type{StaticArrays.SArray{Tuple{S},T,1,S} where T where S}, ::Array{Int64,1}) at /Users/solver/.julia/v0.6/StaticArrays/src/convert.jl:17

It should say something like "Converting vectors to static vectors is not allowed to avoid type instability. Instead, use ...".

[andyferris]

BTW, this is from a subtyping bug in Julia:

https://github.com/JuliaArrays/StaticArrays.jl/blob/master/src/traits.jl#L35-L49

I guess we could work around this and add messages to SArray, SMatrix, SizedArray, MVector, etc. I kind of expected that one to be fixed before they released v0.6, but oh well.

[c42f]

We now have the following error which solves this issue:

julia> convert(SVector, [1,2])
ERROR: The size of type `SArray{Tuple{S},T,1,S} where T where S` is not known.

If you were trying to construct (or `convert` to) a `StaticArray` you
may need to add the size explicitly as a type parameter so its size is
inferrable to the Julia compiler (or performance would be terrible). For
example, you might try

    m = zeros(3,3)
    SMatrix(m)      # this error
    SMatrix{3,3}(m) # correct - size is inferrable