EST-194: auto-initialize epochdate, guard for npe when calculate; cosmetic changes

Auto-initialize epoch date
* EstatioSettingsService#fetchEpochDate will automatically install epoch date if need be
  * using ApplicationSettingKeys
  * registered ApplicationSettingsServiceForEstatio, rather than ApplicationSettingsServiceForJdo
* EstatioSettingsServiceJdo removed, folded up into EstatioSettingsService
* EstatioSettingsService methods now hidden
* EstatioAdministrationService
  - reordered methods, new methods to delegate to Invoices for numerator stuff

calculation guard
* LocalDateInterval#overlap NOW has a guard in it

cosmetic stuff
* Api titled "Migration"
* ChargeGroups#findChargeGroupByReference -> #findChargeGroup
* Charges#findChargeByReference -> #findCharge
* Currencies#findCurrencyByReference -> #findCurrency
* Countries#findCountryByReference -> #findCountry
* States#findStatesByReference -> #findStates, no longer in svc menu, renamed
* Indices#findIndexByReference REMOVED, since only one Index
* Invoices#toBeXxx -> #findToBeXxx
* Tax#rates render eagerly

* UserSettingsServiceForEstatio methods are now hidden
* ApplicationSettingsServiceForEstatio methods are now hidden, delegated to by EstatioSettingsService (itself also hidden)
* Invoices methods to create numerator etc now hidden, delegated to by admin service

* Leases#calculate, now has defaults for parameters
  * ClockService#beginningOfQuarter

other
* reordered isis.services key in isis.properties
* javadoc