Support BigFloat Polynomials

[jverzani]

In #297 support for roots and fitting using BigFloat polynomials was added just by adding a dependency on GenericLinearAlgebra. Unfortunately, this little change causes downstream breakage (JuliaLinearAlgebra/GenericLinearAlgebra.jl#71 (comment)). Until that is resolved, removing this dependency seems prudent.

The support can be had just by the user loading that package; the addition of the dependency just made life easier and more predictable.

p = Polynomial(big.([1.0,2,3,4]))
roots(p) # error
using GenericLinearAlgebra
roots(p) # all fine now...