Constructor for gmx_ekindata_t

Constructor for gmx_ekindata_t

Moved init_ekin_data from tgroup header/source to group and made
it the constructor for gmx_ekindata_t. Also made gmx_ekindata_t a
class since the number of threads is only needed by constructor
and destructor and can thus be private. Further refactoring of
members of gmx_ekindata_t would eliminate the need for the nthreads
data member entirely. Other members we not made private to avoid a
ripple of changes adding "()" to numerous lines in several files.