domain checking

[tpapp]

Evaluation of basis functions and linear combinations should support various forms of domain checking, eg

1. no checking (as currently done), this will just error because of acos for Chebyshev
2. strict checking (terminate with an informative DomainError
3. "soft" extrapolation: when within a small distance of boundaries, just use the boundary value (mostly for dealing with floating point error)

Cf #7, a refactor should implement both.

[tpapp]

We no longer use acos internally, so this is less of an issue. With arguments near the boundary, the error is nicely bounded, so it should not matter. Perhaps document this though.