Improve error messages during casting operations.

[YohannDudouit]

This PR splits MFEM_ASSERT in the memory manager to give more informative messages when an error occurs.

PR	Author	Editor	Reviewers	Assignment	Approval	Merge
#1420	@YohannDudouit	@tzanio	@camierjs + @v-dobrev	04/15/20	⌛due 05/13/20	⌛due 05/20/20

[camierjs]

All the debug tests are failing because of the logical changes in the MFEM_ASSERT.

[YohannDudouit]

Ah! I didn't mean to change the logic. I'll try to fix this, thank you.

[tzanio]

Added more time for review and fixes.

[camierjs]

It's a really good idea to move toward making this class 'friendlier'.

Declaring one GridFunction and doing a HostRead just after it will assert an error in DEBUG mode. The reason can be understood, be I don't think it's so straightforward.

It might not be the place to address these concerns here, but as we are interfacing with external user's code and library, I have the feeling that an effort should be made on this front-end to make it easier to debug.

[v-dobrev]

Declaring one GridFunction and doing a HostRead just after it will assert an error in DEBUG mode.

I don't see this: why do you think this will fail?