Add protection to ensure that preComputedMap has the same NSIDE as requested

[rbiswas4]

In the method of instantiating HealpixTiles using preComputedMap, it is possible to instantiate HealpixTiles with a NSIDE different from the one used to write preComputedMap. There should be conditionals to ensure that different values result in raising a ValueError. The check could be related to checking the number of distinct ipix values in the preComputedMap. The problem is the number of distinct values may be smaller than what is obtained from nside2npix because for example the pixels associated with the Northern Hemisphere will not be represented. So, this requires some form of additional information being written out to preComputedMap. Thus this also needs some work in OpSimSummary. That is represented in https://github.com/rbiswas4/OpSimSummary/issues/53