Fix bound_truncation for TaylorModel1 with TaylorN{T} coeffs

[lbenet]

Fixes #135

[lbenet]

With this PR, the example in #135 yields:

julia> using TaylorModels;

julia> xN, yN = set_variables("x y", order=4, numvars=2);

julia> xtm1_order5 = exp( TaylorModel1( Taylor1(xN, 5), 0..0, 0..0, -0.5 .. 0.5) )
  1.0 + 1.0 x + 0.5 x² + 0.16666666666666666 x³ + 0.041666666666666664 x⁴ + [-4.63936, 3.17106]

julia> xtm1_order4 = TaylorModel1( Taylor1(xN, 4), 0..0, 0..0, -0.5 .. 0.5)
  1.0 x + [0, 0]

julia> new_xtm1_order4, _ = TaylorSeries.fixorder(xtm1_order5, xtm1_order4)
(  1.0 + 1.0 x + 0.5 x² + 0.16666666666666666 x³ + 0.041666666666666664 x⁴ + [-4.63936, 3.17106],   1.0 x + [0, 0])

julia> exp(xtm1_order4)
  1.0 + 1.0 x + 0.5 x² + 0.16666666666666666 x³ + 0.041666666666666664 x⁴ + [-4.61679, 3.16968]

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Details

• 0 of 6 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 65.091%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/auxiliary.jl	0	6	0.0%
<!--	Total:	0	6	0.0%	-->

Totals
Change from base Build 1930417175:	-0.1%
Covered Lines:	1005
Relevant Lines:	1544

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💛 - Coveralls

[lbenet]

cc @AnderGray