Part of #607

So that updates in one thread are seen by others.

Part of #607

Move epochs and getUt1(EOPHistory) to abstract class. Removed
getUt1(EOPHistory) from the TimeScales interface since it is documented
as only being for expert use and added it to LazyLoadedTimeScales for
compatibility. Updated many uses of getUt1(EOPHistory) to use the other
getter for UT1.

Part of #607

Added Frames.of(...) which creates a set a frames using the EOP from the
provided TimeScales. Also added a test case.

Part of #607

In preparation for allowing other ways to load EOP.

Part of #607

Part of #607

Added an implementation of TimeScales where the user is given more
responsibility and power to control loading leap seconds ant EOP.

Part of #607

Extracted an interface for determining the ITRF version of EOP data so
that multiple implementations can be used.

Part of #607

Used factory methods so that their containing classes and types are not
exposed.

Part of #607

And updated several existing parsers to use it. Did not update the
Bulletin A parser as it uses data from multiple files when parsing.

Part of #607

Added an interface for parsing leap seconds. Made the parsers for
tai-utc.dat and utc-tai.history files public.

Part of #607

Evan's contributions are in the public domain, as per his status and
CLA.

Allow the user to specify the data context.

Part of #607

Add method for IRNSS epoch to TimeScales, updated other new code to
reflect changes made.

Part of #607

Remove TODO, add missing doc, create a separate constructor for
SaastamoinenModel that does not use the default data context, replace
references to the default data context in MagneticFieldDetector

Part of #607

Fixes issue #625

Part of #607

Part of #607

Change already modified constructor to accept a DataContext.

Part of #607

Overload constructor.

Part of #607

Part of #607

Used already available data context when parsing files.

Part of #607

Overloaded selectExtrapolator() and some protected constructors of
public classes.

Part of #607

Already had alternative methods. Update javadoc.

Part of #607