Fix Base.one(::Angle2d) method and etc.

[hyrodium]

This PR fixes #194.

Before this PR

julia> one(Angle2d(0.1))
ERROR: MethodError: Cannot `convert` an object of type NTuple{4, Float64} to an object of type Float64
Closest candidates are:
  convert(::Type{T}, ::Base.TwicePrecision) where T<:Number at twiceprecision.jl:250
  convert(::Type{T}, ::AbstractChar) where T<:Number at char.jl:180
  convert(::Type{T}, ::CartesianIndex{1}) where T<:Number at multidimensional.jl:136
  ...
Stacktrace:
 [1] Angle2d{Float64}(theta::NTuple{4, Float64})
   @ Rotations ~/.julia/dev/Rotations/src/core_types.jl:163
 [2] _construct_sametype
   @ ~/.julia/dev/StaticArrays/src/linalg.jl:97 [inlined]
 [3] _one(s::Size{(2, 2)}, m_or_SM::Angle2d{Float64})
   @ StaticArrays ~/.julia/dev/StaticArrays/src/linalg.jl:106
 [4] one(m::Angle2d{Float64})
   @ StaticArrays ~/.julia/dev/StaticArrays/src/linalg.jl:99
 [5] top-level scope
   @ REPL[10]:1

julia> one(RotXY(1,2))
ERROR: Cannot construct a two-axis rotation from a matrix
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:33
 [2] RotXY
   @ ~/.julia/dev/Rotations/src/euler_types.jl:240 [inlined]
 [3] _construct_sametype
   @ ~/.julia/dev/StaticArrays/src/linalg.jl:97 [inlined]
 [4] _one(s::Size{(3, 3)}, m_or_SM::RotXY{Float64})
   @ StaticArrays ~/.julia/dev/StaticArrays/src/linalg.jl:106
 [5] one(m::RotXY{Float64})
   @ StaticArrays ~/.julia/dev/StaticArrays/src/linalg.jl:99
 [6] top-level scope
   @ REPL[29]:1

After this PR

julia> one(Angle2d(0.1))
2×2 Angle2d{Float64} with indices SOneTo(2)×SOneTo(2)(0.0):
 1.0  -0.0
 0.0   1.0

julia> one(RotXY(1,2))
3×3 RotXY{Float64} with indices SOneTo(3)×SOneTo(3)(0.0, 0.0):
  1.0  0.0   0.0
  0.0  1.0  -0.0
 -0.0  0.0   1.0

[codecov]

Codecov Report

Merging #195 (43077e8) into master (15d1c32) will decrease coverage by 0.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
- Coverage   86.12%   85.92%   -0.21%     
==========================================
  Files          14       14              
  Lines        1341     1343       +2     
==========================================
- Hits         1155     1154       -1     
- Misses        186      189       +3     

Impacted Files	Coverage Δ
src/core_types.jl	90.44% <100.00%> (+0.14%)	⬆️
src/euler_types.jl	91.68% <0.00%> (-0.74%)	⬇️

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