This patch extends the LagrangeBasis class to also allow to select
the polynomial order at run-time.  For this, the integer template
parameter 'k' (the order) is made optional, with a default value of -1.
If k is explicitly set to something non-negative, then the basis
implementation behaves as before.  If k is indeed -1 then the basis
object needs to be constructed by a new constructor that accepts
a run-time order.

The changes to LagrangeBasis are completely backward-compatible.
I did not do any measurements of the performance, but as the switch
between run-time and compile-time order is based on a simple
compile-time criterion I expect decent compilers to optimize away
all new code if a compile-time order is selected.